Melt-stable lactide polymer nonwoven fabric and process for manufacture thereof

ABSTRACT

A nonwoven fabric comprised of a lactide polymer. The lactide polymer comprises a plurality of poly(lactide) polymer chains, residual lactide in concentration of less than about 2 percent and water in concentration of less than about 2000 parts-per-million. A process for manufacturing a nonwoven fabric with the lactide polymer composition is also disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuing application of U.S. patentapplication Ser. No. 08/328,550 filed Oct. 25, 1994 and which has issuedas U.S. Pat. No. 5,525,706 on Jun. 11, 1996. The parent application(Ser. No. 08/328,550) was a file-wrapper continuation of Ser. No.08/071,590 filed Jun. 2, 1993 now abandoned which was acontinuation-in-part of U.S. application Ser. No. 07/955,690 filed Oct.2, 1992 and now U.S. Pat. No. 5,338,822. The parent application (Ser.No. 08/328,550) is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nonwoven fabric comprising amelt-stable, biodegradable lactide polymer composition and a process formanufacturing such nonwoven fabrics from a melt-stable, biodegradablelactide polymer.

2. Description of the Prior Art

The need for and uses of nonwoven fabrics have increased tremendously inrecent years. Production of nonwoven roll goods was estimated at 2.5billion pounds in 1992. Nonwoven fabrics are presently used forcoverstock, interlinings, wipes, carrier sheets, furniture and beddingconstruction, filtration, apparel, insulation, oil cleanup products,cable insulating products, hospital drapes and gowns, batteryseparators, outerwear construction, diapers and feminine hygieneproducts.

There are basically three different manufacturing industries which makenonwovens; the textile, paper and extrusion industries. The textileindustry garnets, cards or aerodynamically forms textile fibers intooriented webs. The paper industry employs technology for converting drylaid pulp and wet laid paper systems into nonwoven fabrics. Theextrusion industry uses at least three methods of nonwoven manufacture,those being the spunbond, melt blown and porous film methods. The meltblown method involves extruding a thermoplastic resin through a needlethin die, exposing the extruded fiber to a jet of hot air and depositingthe "blown" fiber on a conveyor belt. These fibers are randomlyorientated to form a web. The spunbond method also utilizes a needlethin die, but orients or separates the fibers in some manner. The porousfilm method employs both slit and annular dies. In one method, a sheetis extruded and drawn, fibrillization occurs and a net-like fabricresults.

A problem associated with current nonwoven materials is that recyclingof the article containing the nonwoven fabric is generally not costeffective. In addition, disposal generally involves creatingnon-degradable waste. A vivid example is the disposal of diapers.Disposable diapers rely heavily on the use of nonwovens in theirconstruction. Millions of diapers are disposed of each year. Thesedisposable diapers end up in landfills or compost sites. The public isbecoming increasingly alarmed over diapers that are not constructed ofbiodegradable material. In order to address the public's concern overthe environment, diaper manufacturers are turning to biodegradablematerials for use in their diapers. Currently, biodegradable materialsmade from starch based polymers, polycaprolactones, polyvinyl alcohols,and polyhydroxybutyrate-valerate-copolymers are under consideration fora variety of different uses in the disposable article market. However,to date, there has not been a satisfactory nonwoven fabric made from abiodegradable material which has properties that can withstand thepresent requirements of nonwoven fabrics.

Although not believed to be known as a precursor for nonwoven fabric,the use of lactic acid and lactide to manufacture a biodegradablepolymer is known in the medical industry. As disclosed by Nieuwenhuis etal. (U.S. Pat. No. 5,053,485), such polymers have been used for makingbiodegradable sutures, clamps, bone plates and biologically activecontrolled release devices. Processes developed for the manufacture ofpolymers to be utilized in the medical industry have incorporatedtechniques which respond to the need for high purity andbiocompatability in the final product. These processes were designed toproduce small volumes of high dollar-value products, with less emphasison manufacturing cost and yield.

In order to meet projected needs for biodegradable packaging materials,others have endeavored to optimize lactide polymer processing systems.Gruber et al. (U.S. Pat. No. 5,142,023) disclose a continuous processfor the manufacture of lactide polymers with controlled optical purityfrom lactic acid having physical properties suitable for replacingpresent petrochemical-based polymers.

Generally, manufacturers of polymers utilizing processes such as thosedisclosed by Gruber et al. will convert raw material monomers intopolymer beads, resins or other pelletized or powdered products. Thepolymer in this form may then be then sold to end users who convert,i.e., extrude, blow-mold, cast films, blow films, thermoform,injection-mold or fiber-spin the polymer at elevated temperatures toform useful articles. The above processes are collectively referred toas melt-processing. Polymers produced by processes such as thosedisclosed by Gruber et al., which are to be sold commercially as beads,resins, powders or other non-finished solid forms are generally referredto collectively as polymer resins.

Prior to the present invention, it is believed that there has been nodisclosure of a combination of composition control and melt stabilityrequirements which will lead to the production of commercially viablelactide polymer nonwoven fabrics.

It is generally known that lactide polymers or poly(lactide) areunstable. The concept of instability has both negative and positiveaspects. A positive aspect is the biodegradation or other forms ofdegradation which occur when lactide polymers or articles manufacturedfrom lactide polymers are discarded or composted after completing theiruseful life. A negative aspect of such instability is the degradation oflactide polymers during processing at elevated temperatures as, forexample, during melt-processing by end-user purchasers of polymerresins. Thus, the same properties that make lactide polymers desirableas replacements for non-degradable petrochemical polymers also createundesirable effects during processing which must be overcome.

Lactide polymer degradation at elevated temperature has been the subjectof several studies, including: I. C. McNeill and H. A. Leiper, PolymerDegradation and Stability, vol. 11, pp. 267-285 (1985); I. C. McNeil andH. A. Leiper, Polymer Degradation and Stability, vol. 11, pp. 309-326(1985); M. C. Gupta and V. G. Deshmukh, Colloid & Polymer Science, vol.260, pp. 308-311 (1982), M. C. Gupta and V. G. Deshmukh, Colloid &Polymer Science, vol. 260, pp. 514-517 (1982); Ingo Luderwald, Dev.Polymer Degradation, vol. 2, pp. 77-98 (1979); Domenico Garozzo, MarioGiuffrida, and Giorgio Montaudo, Macromolecules, vol. 19, pp. 1643-1649(1986); and, K. Jamshidi, S. H. Hyon and Y. Ikada, Polymer, vol. 29, pp.2229-2234 (1988).

It is known that lactide polymers exhibit an equilibrium relationshipwith lactide as represented by the reaction below: ##STR1##

No consensus has been reached as to what the primary degradationpathways are at elevated processing temperatures. One of the proposedreaction pathways includes the reaction of a hydroxyl end group in a"back-biting" reaction to form lactide. This equilibrium reaction isillustrated above. Other proposed reaction pathways include: reaction ofthe hydroxyl end group in a "back-biting" reaction to form cyclicoligomers, chain scission through hydrolysis of the ester bonds, anintramolecular beta-elimination reaction producing a new acid end groupand an unsaturated carbon-carbon bond, and radical chain decompositionreactions. Regardless of the mechanism or mechanisms involved, the factthat substantial degradation occurs at elevated temperatures, such asthose used by melt-processors, creates an obstacle to use of lactidepolymers as a replacement for petrochemical-based polymers. It isapparent that degradation of the polymer during melt-processing must bereduced to a commercially acceptable rate while the polymer maintainsthe qualities of biodegradation or compostability which make it sodesirable. It is believed this problem has not been addressed prior tothe developments disclosed herein.

As indicated above, poly(lactide)s have been produced in the past, butprimarily for use in medical devices. These polymers exhibitbiodegradability, but also a more stringent requirement of beingbioresorbable or biocompatible. As disclosed by M. Vert, Die IngwandteMakromolekulare Chemie, vol. 166-167, pp. 155-168 (1989), "The use ofadditives is precluded because they can leach out easily in body fluidsand then be recognized as toxic, or, at least, they can be the source offast aging with loss of the properties which motivated their use.Therefore, it is much more suitable to achieve property adjustmentthrough chemical or physical structure factors, even if aging is still aproblem." Thus, work aimed at the bioresorbable or biocompatible marketfocused on poly(lactide) and blends which did not include any additives.

Other disclosures in the medical area include Nieuwenhuis (EuropeanPatent No. 0 314 245), Nieuwenhuis (U.S. Pat. No. 5,053,485),Eitenmuller (U.S. Pat. No. 5,108,399), Shinoda (U.S. Pat. No.5,041,529), Fouty (Canadian Patent No. 808,731), Fouty (Canadian PatentNo. 923,245), Schneider (Canadian Patent No. 863,673), and Nakamura etal., Bio. Materials and Clinical Applications, Vol. 7, p. 759 (1987). Asdisclosed in these references, in the high value, low volume medicalspecialty market, poly(lactide) or lactide polymers and copolymers canbe given the required physical properties by generating lactide of veryhigh purity by means of such methods as solvent extraction orrecrystallization followed by polymerization. The polymer generated fromthis high purity lactide is a very high molecular weight product whichwill retain its physical properties even if substantial degradationoccurs and the molecular weight drops significantly during processing.Also, the polymer may be precipitated from a solvent in order to removeresidual monomer and catalysts. Each of these treatments add stabilityto the polymer, but clearly at a high cost which would not be feasiblefor lactide polymer compositions which are to be used to replaceinexpensive petrochemical-based polymers in the manufacture of nonwovenproducts.

Furthermore, it is well-known that an increase in molecular weightgenerally results in an increase in a polymer's viscosity. A viscositywhich is too high can prevent melt-processing of the polymer due tophysical/mechanical limitations of the melt-processing equipment.Melt-processing of higher molecular weight polymers generally requiresthe use of increased temperatures to sufficiently reduce viscosity sothat processing can proceed. However, there is an upper limit totemperatures used during processing. Increased temperatures increasedegradation of the lactide polymer, as the previously-cited studiesdisclose.

Jamshidi et al., Polymer, Vol. 29, pp. 2229-2234 (1988) disclose thatthe glass transition temperature of a lactide polymer, T_(g), plateausat about 57° C. for poly(lactide) having a number average molecularweight of greater than 10,000. It is also disclosed that the meltingpoint, T_(m), of poly (L-lactide) levels off at about 184° C. forsemicrystalline lactide polymers having a number average molecularweight of about 70,000 or higher. This indicates that at a relativelylow molecular weight, at least some physical properties of lactidepolymers plateau and remain constant.

Sinclair et al. (U.S. Pat. No. 5,180,765) disclose the use of residualmonomer, lactic acid or lactic acid oligomers to plasticizepoly(lactide) polymers, with plasticizer levels of 2-60 percent. Loomis(U.S. Pat. No. 5,076,983) discloses a process for manufacturing aself-supporting film in which the oligomers of hydroxy acids are used asplasticizing agents. Loomis and Sinclair et al. disclose that the use ofa plasticizer such as lactide or lactic acid is beneficial to producemore flexible materials which are considered to be preferable. Sinclairet al., however, disclose that residual monomer can deposit out onrollers during processing. Loomis also recognizes that excessive levelsof plasticizer can cause unevenness in films and may separate and stickto and foul processing equipment. Thus, plasticizing as recommended,negatively impacts melt-processability in certain applications.

Accordingly, a need exists for a lactide polymer composition which ismelt-stable under the elevated temperatures common to melt-processingresins in the manufacture of nonwoven fabrics. The needed melt-stablepolymer composition must also exhibit sufficient compostability ordegradability after its useful life as a nonwoven fabric. Further, themelt-stable polymer must be processable in existing melt-processingequipment, by exhibiting sufficiently low viscosities at melt-processingtemperatures while polymer degradation and lactide formation remainsbelow a point of substantial degradation and does not cause excessivefouling of processing equipment. Furthermore, the polymer lactide mustretain its molecular weight, viscosity and other physical propertieswithin commercially-acceptable levels through the nonwoven manufacturingprocess. It will be further appreciated that a need also exists for aprocess for manufacturing such nonwoven fabrics. The present inventionaddresses these needs as well as other problems associated with existinglactide polymer compositions and manufacturing processes. The presentinvention also offers further advantages over the prior art, and solvesother problems associated therewith.

SUMMARY OF THE INVENTION

According to the present invention, a nonwoven fabric comprising aplurality of fibers is provided. A first portion of the plurality offibers comprise a melt-stable lactide polymer composition comprising: aplurality of poly(lactide) polymer chains having a number averagemolecular weight of from about 10,000 to about 300,000; lactide in aconcentration of less than about 2 percent by weight; and water in aconcentration of less than about 2,000 parts per million. A process forthe manufacture of the nonwoven fabric is also provided. For thepurposes of the present invention, the nonwoven fabric may bemanufactured from any number of methods and is not to be limited by theparticular method.

Optionally, stabilizing agents in the form of anti-oxidants and waterscavengers may be added. Further, plasticizers, nucleating agents and/oranti-blocking agents may be added. The resultant nonwoven fabric isbiodegradable and may be disposed of in an environmentally soundfashion.

Poly(lactide) is a polymeric material which offers unique advantages asa fiber for nonwovens not only in the biodegradable sense, but in themanufacturing process as well.

Poly(lactide) offers advantages in the formation of the nonwoven fabricin a melt extrusion process. One problem that is sometimes encounteredin the extrusion of fibers into a nonwoven web is poor adhesion of thefibers to one another upon cooling. Two characteristics ofpoly(lactides) lend themselves to enhanced adhesion low viscosity andhigh polarity. Mechanical adhesion, the interlocking of fibers atadjoining points, increases as the viscosity decreases. An advantage ofpoly(lactide) is that the viscosity lends itself well to fiberformation. Thus, poly(lactide) fibers adhere to one another well,resulting in a web with added strength. Also, because the surface istypically polar for most fibers, the high polarity of the poly(lactide)offers many dipole-dipole interactions, further resulting in enhancedadhesion

In melt blown processes, the fibers of the present invention have smalldiameters which are beneficial for many applications. The present fiberscan have diameters of less than about 5 μm.

The fibers of the nonwoven web of the present invention are superior totypical polypropylene nonwoven webs in diaper construction. The typicalconstruction of a diaper comprises an outer, water impervious backsheet, a middle, absorbent layer and an inner layer, which is in contactwith the diaper wearer. The inner layer is typically made from a soft,nonwoven material such as a polypropylene nonwoven web. However,polypropylene, due to its low polarity, has to be surface modified suchthat the urine passes through the inner layer, rather than beingrepelled. A significant advantage of the present invention is that thepolarity of the poly(lactide) (without surface treatment) is ideallysuited such that urine readily passes through the nonwoven web, but isnot absorbed by the layer. Thus, the poly(lactide) web of the presentinvention also may superior inner layer for diaper construction. Thepresent invention may also be employed in incontinent and femininehygiene products.

The nonwoven fabric of the present invention also may be used inpackaging and bagging operations. Food packaging which does not requirewater tight packaging but requires breathability is an example of a useof the present invention. Bags such as leaf or yard bags may also bemade from the nonwoven fabric of the present invention. The fabric isporous to allow air to enter the bag to begin decomposing the leaves.This is advantageous over present leaf bags, which do not allow air topenetrate into the leaf cavity. Further, the present nonwoven fabric,when used as a leaf bag, decomposes along with the leaves, thusminimizing the adverse environmental impact of the leaf bags.

Poly(lactide) processes at lower temperatures which allows the fiber tobe extruded at lower temperatures than traditional polymers. Thisresults in a cost savings to the converter because the extrusionequipment will not require as much power when run at lower temperatures.There is also increased safety associated with lower temperatures.

A significant advantage of poly(lactide) over many nonwoven fabrics usedtoday such as polypropylene is its biodegradability. The continueddepletion of landfill space and the problems associated withincineration of waste have led to the need for development of a trulybiodegradable nonwoven fabric to be utilized as a substitute fornon-biodegradable or partially biodegradable petrochemical-basednonwoven fabrics. Furthermore, a poly(lactide) nonwoven web, unlikeother biodegradable polymers, is believed to not support microbialgrowth. Starch or other biodegradable polymers, when exposed to warm,damp environments, will promote the growth of unhealthy microbes. Thisis undesirable in the diaper industry. Thus the present invention hasyet another advantage over prior biodegradable polymers.

The above described features and advantages along with various otheradvantages and features of novelty are pointed out with particularity inthe claims of the present application. However, for a betterunderstanding of the invention, its advantages, and objects attained byits use, reference should be made to the drawings which form a furtherpart of the present application and to the accompanying descriptivematter in which there is illustrated and described preferred embodimentsof the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, in which like reference numerals indicate correspondingparts or elements of preferred embodiments of the present inventionthroughout the several views;

FIG. 1 is a schematic representation of a preferred process for themanufacture of a melt-stable lactide polymer composition; and

FIG. 2 is a graph showing the equilibrium relationship between lactideand poly(lactide) at various temperatures FIG. 3 is a graph showing therelationship between meso-lactide concentration and energy of melting.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The lactide polymer compositions used in nonwoven fabrics disclosedherein focus on meeting the requirements of the end user melt-processorof a lactide polymer resin such as that produced from a processdisclosed by Gruber et al. However, the present invention is directed toa poly(lactide) fiber and is not limited to the lactide polymercomposition or process of Gruber et al. Any lactide polymer composition,which comes within the scope of this invention, may be used as a fiberfor a nonwoven fabric. As disclosed herein, the problems of degradation,fouling, and lactide formation during melt-processing of lactidepolymers are addressed through suggested ranges of molecular weights andcompositional limits on impurities such as residual monomer, water andcatalyst along with the use of stabilizing agents andcatalyst-deactivating agents.

In general, according to the present invention, a meltstable lactidepolymer nonwoven fabric and a process for manufacturing a melt-stablelactide polymer nonwoven fabric from a melt-stable lactide polymer aredisclosed. Lactide polymers are useful due to their biodegradablenature. Furthermore, lactide polymers are compostable as illustrated inExample 15 below. Applicants believe the hydrolysis of the ester may bethe key to or the first step in degradation of a lactide polymercomposition. The mechanism of degradation is not key to the nonwovenfabric of the present invention, however it must be recognized that suchdegradation makes lactide polymers desirable as replacements forpresently-utilized non-degradable petrochemical-based polymers used fornonwovens.

Applicants have found that the instability of lactide polymers whichleads to the beneficial degradation discussed above also createsprocessing problems. These processing problems include generation oflactide monomer at elevated temperatures and loss in molecular weightbelieved due to chain scission degradation of the ester bonds and otherdepolymerization reactions which are not completely understood. Noconsensus has been reached as to what are the primary degradationpathways at elevated processing temperatures. As previously disclosed,these may include such pathways as equilibrium-driven depolymerizationof lactide polymers to form lactide and chain scission throughhydrolysis of the ester bonds along with other pathways. For purposes ofthe present invention, the exact mechanism of degradation at elevatedtemperatures is not critical.

It is to be understood, however, that degradation of lactide polymers isboth beneficial and detrimental. Benefits derive from degradability whenarticles manufactured from such polymers are discarded. The same orsimilar types of degradation are detrimental if they occur duringprocessing or prior to the end of the article's useful life.

Melt-Processing

It is believed that a manufacturer of lactide polymers from a lactidemonomer will produce a lactide polymer resin which is in the form ofbeads or pellets. The melt-processor will convert the resin to a fiberfor a nonwoven fabric by elevating the temperature of the resin above atleast its glass transition temperature but normally higher and extrudingthe fiber into a nonwoven fabric. It is to be understood that theconditions of elevated temperature used in melt-processing causedegradation of lactide polymers during processing. Degradation undermelt-processing conditions is shown experimentally in Example 7 based onequilibrium, Example 10 based on catalyst concentration, Example 11based on catalyst activity, Example 13 based on use of stabilizers andExample 14 based on moisture content. As can be seen in these examples,it is understood that several factors appear to affect the rate ofdegradation during melt-processing. Applicants have addressed thesefactors in a combination of compositional requirements and the additionof stabilizing or catalyst-deactivating agents to result in a polymer oflactide which is melt-stable.

In addition, melt-processing frequently produces some proportion oftrimmed or rejected material. Environmental concerns and economicalefficiencies dictate that this material be reused, typically byregrinding and adding back the material into the polymer feed. Thisintroduces additional thermal stress on the polymer and increases theneed for a melt-stable polymer composition.

Melt Stability

The lactide polymers of the present invention are meltstable. By"melt-stable" it is meant generally that the lactide polymer, whensubjected to melt-processing techniques, adequately maintains itsphysical properties and does not generate by-products in sufficientquantity to foul or coat processing equipment. The melt-stable lactidepolymer exhibits reduced degradation and/or reduced lactide formationrelative to known lactide polymers. It is to be understood thatdegradation will occur during melt-processing. The compositionalrequirements and use of stabilizing agents as disclosed herein reducesthe degree of such degradation to a point where physical properties arenot significantly affected by melt-processing and fouling by impuritiesor degradation by-products such as lactide does not occur. Furthermore,the melt-stable polymer should be melt-processable in melt-processingequipment such as that available commercially. Further, the polymer willpreferably retain adequate molecular weight and viscosity. The polymershould preferably have sufficiently low viscosity at the temperature ofmelt-processing so that the extrusion equipment may create an acceptablenonwoven fabric. The temperature at which this viscosity is sufficientlylow will preferably also be below a temperature at which substantialdegradation occurs.

Polymer Composition

The melt-stable lactide polymer nonwoven fabric of the present inventioncomprises a plurality of poly(lactide) polymer chains having a numberaverage molecular weight from about 10,000 to about 300,000. In apreferred composition for a melt blown nonwoven, the number averagemolecular weight ranges from about 15,000 to about 100,000. In the mostpreferred composition, the number average molecular weight ranges fromabout 20,000 to about 80,000. In a spunbond nonwoven fabric, thepreferred number average molecular weight range is from about 50,000 toabout 250,000. In a most preferred embodiment, the number averagemolecular weight range is from about 75,000 to about 200,000.

As detailed in Example 9, it appears that the physical properties suchas modulus, tensile strength, percentage elongation at break, impactstrength, flexural modulus, and flexural strength remain statisticallyconstant when the lactide polymer samples are above a thresholdmolecular weight. The lower limit of molecular weight of the polymercompositions of the present invention is set at a point above thethreshold of which a fiber has sufficient diameter and density. In otherwords, the molecular weight cannot be lower than is necessary to achievea targeted fiber diameter and density. As detailed in Example 22, thereis a practical upper limit on molecular weight based on increasedviscosity with increased molecular weight. In order to melt-process ahigh molecular weight lactide polymer, the melt-processing temperaturemust be increased to reduce the viscosity of the polymer. As pointed outin the Examples, the exact upper limit on molecular weight must bedetermined for each melt-processing application in that requiredviscosities vary and residence time within the melt-processing equipmentwill also vary. Thus, the degree of degradation in each type ofprocessing system will also vary. Based on the disclosure of Example 22,it is believed that one could determine the suitable molecular weightupper limit for meeting the viscosity and degradation requirements inany application.

Lactide polymers can be in either an essentially amorphous form or in asemi-crystalline form. For various applications it will be desirable tohave the polymer in one of these configurations. As detailed in Example24, the desired range of compositions for semi-crystalline poly(lactide)is less than about 12 percent by weight meso-lactide and the remainingpercent by weight either L-lactide or D-lactide, with L-lactide beingmore readily available. A more preferred composition contains less thanabout 9 percent by weight meso-lactide with the remainder beingsubstantially all L-lactide.

For applications where an amorphous polymer is desired, the preferredcomposition of the reaction mixture is above 9 percent by weightmeso-lactide and a more preferred composition contains above 12 percentby weight meso-lactide lactide with the remaining lactide beingsubstantially all L-lactide mixture, or D-lactide can be used to controlthe potential crystallinity in a predominantly L-lactide mixture.

Addition of even small amounts of meso-lactide to the polymerizationmixture results in a polymer which is even slower to crystallize thanpolymerization mixtures having lesser amounts of meso-lactide, asdetailed in Example 23. Beyond about 12% meso content the polymerremains essentially amorphous following a typical annealing procedure.This contrasts with the behavior of D,L-lactide, which can be added at aconcentration of 20 percent to the polymerization mixture and stillproduce a semi-crystalline polymer following the annealing procedure.These results are detailed in Example 24.

There are three main methods to increase the rate of crystallization.One is to increase chain mobility at low temperatures, by adding forexample, a plasticizing agent. The plasticizer must be selectedcarefully, however, and preferably will be of limited compatibility sothat it will migrate to the amorphous phase during crystallization.Dioctyl adipate is an example of a plasticizer which helpscrystallization rates in poly(lactide), as detailed in Example 25. Asecond method to increase the rate of crystallization is to add anucleating agent, as detailed in Example 26. A third method is to orientthe polymer molecules. Orientation can be accomplished by drawing duringfilm casting, drawing of fibers, blowing films, stretching of film orsheet after it is cast (in multiple directions, if desired), or by theflow of polymer through a small opening in a die. The alignmentgenerated helps to increase the rate of crystallization, as detailed inExample 27.

Heat setting may also be employed to increase the degree ofcrystallinity in the fibers. Heat setting involves exposing the fabricto elevated temperatures, as shown in Plastics Extrusion Technology, F.Hensen (ed), Hanser Publishers, New York, 1988, pp 308, 324. It ispreferred to heat set with the fiber or nonwoven fabric under tension toreduce shrinkage during the setting process.

Applicants recognize that an essentially amorphous lactide polymer mayhave some crystallinity. Crystalline poly L-lactide exhibits anendotherm of roughly 92 Joules per gram at its melting temperature of170°-190° C. The melting point changes with composition. The degree ofcrystallinity is roughly proportional to the endotherm on melting. Forpurposes of the present invention, it is meant by an amorphous ornon-crystalline poly(lactide) to be a poly(lactide) or lactide polymerwhich exhibits a melting endotherm of less than about 10 Joules per gramin the temperature range of about 130°-200° C. Semi-crystallinepoly(lactide) exhibits a melting endotherm above about 10 joules pergram.

The residual monomer concentration in the melt-stable lactide polymercomposition is less than about 2 percent by weight. In a preferredcomposition, the lactide concentration is less than about 1 percent byweight, and a most preferred composition has less than about 0.5 percentby weight of lactide. Contrary to disclosures in the art, Applicantshave found that the monomer cannot be used as a plasticizing agent inthe resin of the present invention due to significant fouling of theextrusion equipment. As detailed in Example 16, it is believed the lowlevels of monomer concentration do not plasticize the final polymer.

The water concentration within the melt-stable lactide polymercomposition is less than about 2,000 parts-per-million. Preferably thisconcentration is less than 500 parts-per-million and most preferablyless than about 100 parts-per-million. As detailed in Example 14, thepolymer melt-stability is significantly affected by moisture content.Thus, the melt-stable polymer of the present invention must have thewater removed prior to melt processing. Applicants recognize that waterconcentration may be reduced prior to processing the polymerized lactideto a resin. Thus, moisture control could be accomplished by packagingsuch resins in a manner which prevents moisture from contacting thealready-dry resin. Alternatively, the moisture content may be reduced atthe melt-processor's facility just prior to the melt-processing step ina dryer. Example 14 details the benefit of drying just prior tomelt-processing and also details the problems encountered due to wateruptake in a polymer resin if not stored in a manner in which moistureexposure is prevented or if not dried prior to melt-processing. Asdetailed in these examples, Applicants have found that the presence ofwater causes excessive loss of molecular weight which may affect thephysical properties of the melt-processed polymer.

In a preferred composition of the present invention, a stabilizing agentis included in the polymer formulation to reduce degradation of thepolymer during production devolatilization, drying and melt processingby the end user. The stabilizing agents recognized as useful in thepresent nonwoven fibers may include antioxidants and/or waterscavengers. Preferred antioxidants are phosphite-containing compounds,hindered phenolic compounds or other phenolic compounds. Theantioxidants include such compounds as trialkyl phosphites, mixedalkyl/aryl phosphites, alkylated aryl phosphites, sterically hinderedaryl phosphites, aliphatic spirocyclic phosphites, sterically hinderedphenyl spirocyclics, sterically hindered bisphosphonites, hydroxyphenylpropionates, hydroxy benzyls, alkylidene bisphenols, alkyl phenols,aromatic amines, thioethers, hindered amines, hydroquinones and mixturesthereof. As detailed in Example 13, many commercially-availablestabilizing agents have been tested and fall within the scope of thepresent melt-stable lactide polymer nonwoven fabric composition.Biodegradable antioxidants are particularly preferred.

The water scavengers which may be utilized in preferred embodiments ofthe melt-stable lactide polymer nonwoven fiber include: carbodiimides,anhydrides, acyl chlorides, isocyanates, alkoxy silanes, and desiccantmaterials such as clay, alumina, silica gel, zeolites, calcium chloride,calcium carbonate, sodium sulfate, bicarbonates or any other compoundwhich ties up water. Preferably the water scavenger is degradable orcompostable. Example 19 details the benefits of utilizing a waterscavenger.

In a preferred composition of the present invention, a plasticizer isincluded in the polymer formulation to improve the nonwoven fiberquality of the lactide polymer. More particularly, plasticizers reducethe melt viscosity at a given temperature of poly(lactide), which aidesin processing and extruding the polymer at lower temperatures and mayimprove flexibility and reduce cracking tendencies of the finishedfabric. Plasticizers also lower the melt viscosity of poly(lactide),thereby making it easier to draw-down the fibers to a small diameter.

A plasticizer is useful in concentration levels of about 1 to 35percent. Preferably, a plasticizer is added at a concentration level ofabout 5 to 25 percent. Most preferably, a plasticizer is added at aconcentration level of about 8 to 25 percent.

Selection of a plasticizing agent requires screening of many potentialcompounds and consideration of several criteria. For use in abiodegradable nonwoven fabric the preferred plasticizer is to bebiodegradable, non-toxic and compatible with the resin and relativelynonvolatile.

Plasticizers in the general classes of alkyl or aliphatic esters, ether,and multi-functional esters and/or ethers are preferred. These includealkyl phosphate esters, dialkylether diesters, tricarboxylic esters,epoxidized oils and esters, polyesters, polyglycol diesters, alkylalkylether diesters, aliphatic diesters, alkylether monoesters, citrateesters, dicarboxylic esters, vegetable oils and their derivatives, andesters of glycerine. Most preferred plasticizers are tricarboxylicesters, citrate esters, esters of glycerine and dicarboxylic esters.Citroflex A4® from Morflex is particularly useful. These esters areanticipated to be biodegradable. Plasticizers containing aromaticfunctionality or halogens are not preferred because of their possiblenegative impact on the environment.

For example, appropriate non-toxic character is exhibited by triethylcitrate, acetyltriethyl citrate, tri-n-butyl citrate, acetyltri-n-butylcitrate, acetyltri-n-hexyl citrate, n-butyltri-n-hexyl citrate anddioctyl adipate. Appropriate compatibility is exhibited byacetyltri-n-butyl citrate and dioctyl adipate. Other compatibleplasticizers include any plasticizers or combination of plasticizerswhich can be blended with poly(lactide) and are either miscible withpoly(lactide) or which form a mechanically stable blend. Corn oil andmineral oil were found to be incompatible when used alone withpoly(lactide) because of phase separation (not mechanically stable) andmigration of the plasticizer.

Volatility is determined by the vapor pressure of the plasticizer. Anappropriate plasticizer must be sufficiently non-volatile such that theplasticizer stays substantially in the resin formulation throughout theprocess needed to produce the nonwoven fabric. Excessive volatility canlead to fouling of process equipment, which is observed when producingfabrics by melt processing poly(lactide) with a high lactide content.Preferred plasticizers should have a vapor pressure of less than about10 mm Hg at 170° C., more preferred plasticizers should have a vaporpressure of less than 10 mm Hg at 200° C. Lactide, which is not apreferred plasticizer, has a vapor pressure of about 40 mm Hg at 170° C.Example 6 highlights useful plasticizers for the present invention.

In a preferred composition, nucleating agents may be incorporated duringpolymerization. Nucleating agents may include selected plasticizers,finely divided minerals, organic compounds, salts of organic acids andimides and finely divided crystalline polymers with a melting pointabove the processing temperature of poly(lactide). Examples of usefulnucleating agents include talc, sodium salt of saccharin, calciumsilicate, sodium benzoate, calcium titanate, boron nitride, copperphthalocyanine, isotactic polypropylene, low molecular weightpoly(lactide) and polybutylene terephthalate,

In a preferred composition, fillers may be useful to prevent blocking orsticking of layers or rolls of the nonwoven fabric during storage andtransport. Inorganic fillers include clays and minerals, either surfacemodified or not. Examples include talc, diatomaceous earth, silica,mica, kaolin, titanium dioxide, and wollastonite. Preferred inorganicfillers are environmentally stable and non-toxic.

Organic fillers include a variety of forest and agricultural products,either with or without modification. Examples include cellulose, wheat,starch, modified starch, chitin, chitosan, keratin, cellulosic materialsderived from agricultural products, gluten, nut shell flour, wood flour,corn cob flour, and guar gum. Preferred organic fillers are derived fromrenewable sources and are biodegradable. Fillers may be used eitheralone or as mixtures of two or more fillers. Example 5 highlights usefulanti-blocking fillers for the present invention.

Surface treatments may also be used to reduce blocking. Such treatmentsinclude corona and flame treatments which reduce the surface contactbetween the poly(lactide) based fabric and the adjacent surface.

For certain applications, it is desirable for the fabric to be modifiedto alter the water transport properties. Surfactants may be incorporatedinto the web of the present invention to increase the water transportproperties.

Surfactants which are useful can be subdivided into cationic, anionic,and nonionic agents.

With regard to cationic compounds, the active molecule part generallyconsists of a voluminous cation which often contains a long alkylresidue (e.g. a quaternary ammonium, phosphonium or sulfonium salt)whereby the quaternary group can also occur in a ring system (e.g.imidazoline). In most cases, the anion is the chloride, methosulfate ornitrate originating from the quaternization process.

In the anionic compounds, the active molecule part in this class ofcompounds is the anion, mostly an alkyl sulfonate, sulfate or phosphate,a dithiocarbamate or carboxylate. Alkali metals often serve as cations.

Nonionic antistatic agents are uncharged surface-active molecules of asignificantly lower polarity than the above mentioned ionic compoundsand include polyethylene glycol esters or ethers, fatty acid esters orethanolamides, mono- or diglycerides or ethyoxylated fatty amines. Theabove surfactants may also act as antistatic agents, which may bedesirable.

Pigments or color events may also be added as necessary. Examplesinclude titanium dioxide, clays, calcium carbonate, talc, mica, silica,silicates, iron oxides and hydroxide, carbon black and magnesium oxide.

In the manufacture of the melt-stable lactide polymer compositions ofthe present invention, the reaction to polymerize lactide is catalyzed.Many catalysts have been cited in literature for use in the ring-openingpolymerization of lactones. These include but are not limited to: SnCl₂SnBr₂, SnCl₄, SnBr₄, aluminum alkoxides, tin alkoxides, zinc alkoxides,SnO, PbO, Sn (2-ethyl hexanoates), Sb (2-ethyl hexanoates), Bi (2-ethylhexanoates), Na (2-ethyl hexanoates) (sometimes called octoates), Castearates, Mg stearates, Zn stearates, and tetraphenyltin. Applicantshave also tested several catalysts for polymerization of lactide at 180°C. which include: tin(II) bis(2-ethyl hexanoate) (commercially availablefrom Atochem, as Fascat 2003, and Air Products as DABCO T-9), dibutyltindiacetate (Fascat 4200®, Atochem), butyltin tris(2-ethyi hexanoate)(Fascat 9102®, Atochem), hydrated monobutyltin oxide (Fascat 9100®,Atochem), antimony triacetate (S-21, Atochem), and antimonytris(ethylene glycoxide) (S-24, Atochem). Of these catalysts, tin(II)bis-(2-ethyl hexanoate), butyltin tris(2-ethyl hexanoate) and (butyltindiacetate appear to be most effective.

Applicants have found the use of catalysts to polymerize lactidesignificantly affects the stability of the resin product. It appears thecatalyst as incorporated into the polymer also is effective atcatalyzing the reverse depolymerizations reaction. Example 10 detailsthe effect of residual catalyst on degradation. To minimize thisnegative effect, in a preferred composition, the residual catalyst levelin the resin is present in a molar ratio of initial monomer-to-catalystgreater than about 3,000:1, preferably greater than about 5,000:1 andmost preferably greater than about 10,000:1. Applicants believe a ratioof about 20,000:1 may be used, but polymerization will be slow.Optimization of catalyst levels and the benefits associated therewithare detailed in Example 20. Applicants have found that when the catalystlevel is controlled within these parameters, catalytic activity issufficient to polymerize the lactide while sufficiently low to enablemelt-processing without adverse effect when coupled with low residualmonomer level and low water concentration as described above in polymersof molecular weight between 10,000 to about 300,000. It is believed inmost applications the addition of a stabilizing agent may be unnecessaryif catalyst level is optimized.

Applicants have also found that catalyst concentration may be reducedsubsequent to polymerization by precipitation from a solvent. Example 21demonstrates potential catalyst removal by precipitation from a solvent.This produces a resin with reduced catalyst concentration. In analternative embodiment, the catalyst means for catalyzing thepolymerization of lactide to form the poly(lactide) polymer chains whichwas incorporated into the melt-stable lactide polymer composition duringpolymerization is deactivated by including in the melt-stable lactidepolymer composition a catalyst deactivating agent in amounts sufficientto reduce catalytic depolymerization of the poly(lactide) polymerchains. Example 11 details the benefits of utilizing a catalystdeactivating agent. Such catalyst-deactivating agents include hindered,alkyl, aryl and phenolic hydrazides, amides of aliphatic and aromaticmono- and dicarboxylic acids, cyclic amides, hydrazones andbishydrazones of aliphatic and aromatic aldehydes, hydrazides ofaliphatic and aromatic mono- and dicarboxylic acids, bis-acylatedhydrazine derivatives, and heterocyclic compounds. A preferred metaldeactivator is Irganox® MD1024 from Ciba-Geigy. Biodegradable metaldeactivators are particularly preferred.

In an alternative embodiment, the catalyst concentration is reduced tonear zero by utilizing a solid-supported catalyst to polymerize lactide.The feasibility of utilizing such a catalyst is detailed in Example 8.It is believed catalysts which may be utilized include supported metalcatalysts, solid acid catalysts, acid clays, alumina silicates, alumina,silica and mixtures thereof.

In a preferred composition, the catalyst usage and/or deactivation iscontrolled to reduce depolymerization of the poly(lactide) polymerduring melt-processing to less than about 2 percent by weight generationof lactide from a devolatilized sample in the first hour at 180° andatmospheric pressure. More preferably, the amount of lactide generatedis less than about 1 percent by weight in the first hour and mostpreferably less than about 0.5 percent by weight in the first hour.

A preferred melt-stable lactide polymer composition is the reactionproduct of polymerization of lactide at a temperature greater than about160° C. Applicants have found that polymerization at higher temperaturesresult in a characteristically different polymer which is believed tohave improved melt stability due to increased transesterification duringpolymerization. The benefits of higher temperature polymerization aredetailed in Example 12.

Melt-Stable Lactide Polymer Process

The process for the manufacture of a melt-stable lactide polymercomprises the steps of first providing a lactide mixture wherein themixture contains about 0.5 percent by weight to about 50 percent byweight meso-lactide and about 99.5 percent by weight or less L-lactideand/or D-lactide. Such purified lactide stream may be such as thatproduced in the process disclosed by Gruber et al., although the sourceof lactide is not critical to the present invention.

The lactide mixture is polymerized to form a lactide polymer orpoly(lactide) with some residual unreacted monomer in the presence of acatalyst means for catalyzing the polymerization of lactide to formpoly(lactide). Catalysts suitable for such polymerization have beenlisted previously. The concentration of catalysts utilized may beoptimized as detailed in the following examples and discussedpreviously.

In a preferred embodiment, a stabilizing agent, which may be anantioxidant and/or a water scavenger is added to the lactide polymer. Itis recognized that such stabilizing agents may be added simultaneouslywith or prior to the polymerization of the lactide to form the lactidepolymer. The stabilizing agent may also be added subsequent topolymerization.

As previously disclosed, the catalyst usage is adjusted and/ordeactivation agent is added in a sufficient amount to reducedepolymerization of poly(lactide) during melt-processing to less than 2percent by weight generation of lactide from a devolatilized sample inthe first hour at 180° C. and atmospheric pressure. More preferably, thestabilizing agent controls lactide generation to less than 1 percent byweight and most preferably less than 0.5 percent by weight in the firsthour at 180° C. and atmospheric pressure. Alternatively, the control ofcatalyst concentration to optimize the balance between necessarycatalytic activity to produce poly(lactide) versus the detrimentaleffects of catalytic depolymerization or degradation of the lactidepolymer may be utilized to obviate the need for adding a stabilizingagent.

The lactide polymer is then devolatilized to remove unreacted monomerwhich may also be a by-product of decomposition reactions or theequilibrium-driven depolymerization of poly(lactide). Any residual waterwhich may be present in the polymer would also be removed duringdevolatilization, although it is recognized that a separate drying stepmay be utilized to reduce the water concentration to less than about2,000 parts-per-million. The devolatilization of the lactide polymer maytake place in any known devolatilization process. The key to selectionof a process is operation at an elevated temperature and usually underconditions of vacuum to allow separation of the volatile components fromthe polymer. Such processes include a stirred tank devolatilization or amelt-extrusion process which includes a devolatilization chamber and thelike An inert gas sweep is useful for improved devolatization.

In a preferred process for manufacture of a melt-stable lactide polymercomposition, the process also includes the step of adding a molecularweight control agent to the lactide prior to catalyzing thepolymerization of the lactide. For examples molecular weight controlagents include active hydrogen-bearing compounds, such as lactic acid,esters of lactic acid, alcohols, amines, glycols, diols and triols whichfunction as chain-initiating agents. Such molecular weight controlagents are added in sufficient quantity to control the number averagemolecular weight of the poly(lactide) to between about 10,000 and about300,000.

Next referring to FIG. 1 which illustrates a preferred process forproducing a melt-stable lactide polymer composition. A mixture oflactides enters a mixing vessel (3) through a pipeline (1). A catalystfor polymerizing lactide is also added through a pipeline (13). Withinmixing vessel (3) a stabilizing agent may be added through a pipeline(2). A water scavenger may also be added through the pipeline (2). Thestabilized lactide mixture is fed through a pipeline (4) to apolymerization process (5). The polymerized lactide or lactide polymerleaves the polymerization process through a pipeline (6). The stream isfed to a second mixing vessel (8) within which a stabilizing agentand/or catalyst deactivating agent may be added through a pipeline (7).The stabilized lactide polymer composition is then fed to adevolatilization process (10) through a pipeline (9) Volatile componentsleave the devolatilization process through a pipeline (11) and thedevolatilized lactide polymer composition leaves the devolatilizationprocess (10) in a pipeline (12). The devolatilized lactide compositionis fed to a resin-finishing process (14). Within the resin-finishingprocess the polymer is solidified and processed to form a pelletized orgranular resin or bead. Applicants recognize the polymer may besolidified and processed to form resin or bead first, followed bydevolatilization. The resin is then fed to a drying process (16) byconveyance means (15). Within the drying process (16) moisture isremoved as a vapor through pipeline (17). The dried lactide polymerresin leaves the drying process (16) by a conveyance means (18) and isfed to a melt-processing apparatus (19). Within the melt-processingapparatus (19) the resin is converted to a useful article as disclosedabove. The useful article leaves the melt-processing apparatus (19)through a conveyance means (20).

The following examples further detail advantages of the system disclosedherein:

EXAMPLE 1 Melt Spinning of Poly(lactide)

Melt spinning of poly(lactide) having a weight average molecular weightof 140,000, a residual lactide content of about 1.1 percent and anoriginal lactide mixture of about 7 percent by weight meso-lactide wasperformed on a 13 mm. single screw extruder with a gear pump and fittedwith a 7 hole multifilament spinning head. The hole diameter was 0.4 mm.The spinline length, the distance from the spinning head to the take-uproll, was 1.7 meters. Polymer throughput was 1 g/min/hole. Filamentswere drawn down by drawing through a circular aspirator which makes useof high velocity air to apply a force downward on the fibers. Postdrawing of the fibers was also done on a draw stand or heated godet.

The process conditions were varied to find conditions under which fiberscould be made from poly(lactide). Extrusion temperatures were variedfrom 150° to 170° C. and the take up velocity was varied from 500 to6,000 meters/min. Fiber diameters were measured and are shown in Table 1for the various fiber spinning conditions. With a microscope equippedwith a light polarizer, birefringence was measured to assess the extentof polymer orientation within the fiber as a function of spinningconditions, Table 1 shows birefringence as a function of take-upvelocity and fiber diameter as a function of take-up velocityrespectively.

                  TABLE 1                                                         ______________________________________                                        Take-up   Extrusion   Fiber                                                   Velocity  Temperature Diameter Birefringence ×                          (meters/min)                                                                            (°C.)                                                                              (microns)                                                                              1000                                           ______________________________________                                        2674      170         19.0     13.84                                          6179      160         12.5     15.84                                          4656      160         14.4     12.71                                          3177      160         17.6      9.09                                          4656      150         14.4     14.94                                          3421      150         16.8      8.04                                          3117      150         17.6     11.82                                           478      150         45.0      0.91                                          ______________________________________                                    

Fibers collected at a take-up velocity of 478 meters/min were post drawnon a heated godet. This apparatus is a series of rolls, including anunwind roll on the front and a take-up roll on the back. With the takeup roll rotating faster than the unwind roll, the fiber is stretched.The rolls in between the unwind and take-up are heated to a temperatureof 50° C. to soften the polymer and allow the fiber to be drawn.Measuring fiber diameter allows calculation of the draw ratio andbirefringence relates to the degree of orientation of the polymerchains. Table 2 summarizes the drawing data. The data illustrates it ispossible to postdraw the fibers to increase the orientation of thefibers.

                  TABLE 2                                                         ______________________________________                                        Initial   Final        Draw   Birefringence ×                           Diameter  Diameter     Ratio  1000                                            ______________________________________                                        45.00     26.00        3.00   16.64                                           45.00     25.10        3.18   19.73                                           45.00     24.80        3.32   21.94                                           45.00     24.00        3.50   19.58                                           ______________________________________                                    

EXAMPLE 2 Properties of Poly(lactide) Melt Spun Fibers

In an apparatus similar to that used in Example 1, poly(lactide) havinga weight average molecular weight of about 100,000, a residual lactidecontent of less than about 1 percent and an original lactide mixture ofabout 10 percent by weight of meso-lactide was melt spun into a fiber.The optical composition was such that upon annealing, (the sample washeld at 100° C. for 90 minutes, the oven was turned off and was allowedto cool to room temperature), the polymer exhibited an endothermic meltpeak with a peak temperature of 140° C. with an endotherm of 36.1joules/gram.

The poly(lactide) fibers were post drawn as in Example 1. The thermaland mechanical properties are shown in Table 3. The results comparefavorably to standard fiber resins such as polypropylene and nylon. Theelongation and modulus compare favorably to commercial fibers. Further,poly(lactide) exhibited an affinity to crystallize under the conditionsof fiber spinning.

                  TABLE 3                                                         ______________________________________                                        Properties                                                                              Resin   As-Spun  Drawn Polyprop.                                                                            Nylon 6,6                             ______________________________________                                        Melt Temp (°C.)                                                                  133     140      140   170    265                                   Heat of Fusion                                                                             2.4  14.2     26.4  105    xxxxx                                 (J/g)                                                                         Denier    xxxxx   162      57    xxxxx  xxxxx                                 (g/9000 m)                                                                    Tenacity (g/den)                                                                        xxxxx   0.97     2.75     6.5    5.4                                Break Elongation                                                                        xxxxx   165%     38%   34%    20%                                   Young's Modulus                                                                         xxxxx   22       44     68     34                                   (g/den)                                                                       ______________________________________                                    

EXAMPLE 3 Melt Blown Fabrics from Poly(lactide)

On a six inch melt blown nonwoven line equipped with a single screwextruder, poly(lactide) of a weight average molecular weight of about80,000, a residual lactide content of about 0.6 percent, an originallactide mixture of about 9 percent by weight of meso-lactide and a watercontent of about 70 ppm was converted into melt blown nonwoven webs.This process involves feeding resin pellets into a feeding hopper of anextruder having a one inch single screw and extruding molten polymerthrough a die containing many small holes out of which emerges smalldiameter fiber. The fiber diameter is attenuated at the die as the fiberemerges using high velocity hot air. Three inches from the die exit is arotating collection drum on which the fibrous web is deposited andconveyed to a wind up spool. The melt blown line is of standard designas in Malkan et al. (Nonwovens: An Advanced Tutorial, TAPPI press,Atlanta, 1989, pp 101-129). The die used had 121 holes with a diameterof 0.020 inch per hole.

Conditions were varied to find conditions under which poly(lactide)could be made into a useful nonwoven fabric. The die temperature wasvaried from 380° to 480° F., the air temperature was varied from 458° to500° F., the die-to-collector distance varied from 6 to 14 inches andthe air velocity varied from approximately 12 to 18 cu-ft/min/inch web.

The resultant poly(lactide) webs were tested for performance usingstandard tests for melt blown fabrics. The basic weight of all webs was1 oz/sq-yd. Fiber diameter and fiber diameter variability were measuredusing a scanning electronic microscope. Tensile stress-strain propertieswere measured using ASTM method D-1682-64. Bursting strength wasmeasured using the Mullen Bursting Tester and ASTM method D-3387.Filtration efficiency was assessed using an aerosol having 0.1 micronsodium chloride particles. The filtration test involved making a 20gram/liter NaCl solution and making an aerosol of the solution with aconcentration of 100 milligram per cubic meter. The aerosol wasthereafter passed through the fabric at 31 liters/minute. Sensors wereplaced both upstream and downstream of the fabric, with the differencereflecting the amount remaining in the filter. Air permeability, anotherfeature important to filtration, was measured according to ASTM D737-75and reported as cubic feet of air per square feet of fabric per minute.All of these performance measures were compared to standardpolypropylene fabrics. The data illustrates poly(lactide) processes aswell as polypropylene. Poly(lactide) is capable of forming fine or smalldiameter fibers. Fibers having diameters of less than about 5 μm areshown. Further, poly(lactide) nonwoven webs have a high filtrationefficacy as well as good air permeability. The results are shown inTable

                                      TABLE 4                                     __________________________________________________________________________    Sample    1   2   3   4   5   6   7   8   typical PP                          __________________________________________________________________________    Conditions:                                                                   Die temp. °F.                                                                    380 395 395 395 395 395 395 395 480                                 Air temp. °F.                                                                    446 446 446 446 446 460 460 458 500                                 DCD inches                                                                              10  10  10  6   14  10  14  10   12                                 Air Valve rate                                                                          40% 40% 50% 50% 50% 60% 60% 55% 40%                                 Property:                                                                     Air       271 247 175 90  227 103 137 149  50-100                             Permeabil.                                                                    ft3/ft2/min                                                                   Bursting strength, psi                                                                  8.3 9.3 9.6 7.6 8.6 8.8 10.6                                                                              10.0                                                                               6-10                               Filtration Eff. %                                                                       38.9                                                                              37.3                                                                              43.2                                                                              60.0                                                                              47.7                                                                              65.3                                                                              59.0                                                                              47.0                                                                              25-60                               Peak Load, lb/in.                                                                       2.10                                                                              2.15                                                                              2.30                                                                              4.77                                                                              1.00                                                                              3.26                                                                              1.50                                                                              2.80                                                                              0.8-3.5                             Peak Elongation, %                                                                      3.10                                                                              2.80                                                                              3.70                                                                              3.90                                                                              2.50                                                                              5.10                                                                              3.00                                                                              4.40                                                                              10-30                               Av. Fiber Dia. μm                                                                    3.83                                                                              xxxx                                                                              2.88                                                                              xxxx                                                                              xxxx                                                                              xxxx                                                                              3.41                                                                              xxxx                                                                              2-4                                 C.V. % Fiber Dia.                                                                       32.61                                                                             xxxx                                                                              37.77                                                                             xxxx                                                                              xxxx                                                                              xxxx                                                                              37.03                                                                             xxxx                                                                              25-40                               __________________________________________________________________________

EXAMPLE 4 Melt Blow Nonwovens made from Poly(lactide)

Melt blown fabrics were made with poly(lactide) using the same equipmentand procedure as in Example 3. The extrusion temperature was 320° F.,screw speed was 8 rpm, die temperature was 315° F., air temperature andair velocity were at 400° F. and 12 cu ft/min/inch web respectively.Die-to-collector distance was 13 inches.

The poly(lactide) used in this test had a weight average molecularweight of about 66,000, a residual lactide concentration of about 1.3%and an original lactide mixture of about 9 percent by weight ofmeso-lactide. This lower molecular weight resulted in softer nonwovenfabrics than Example 3 and had good hand. Fiber diameters were measuredand found to be 11.57 pm. Other tests done on this fabric was the airpermeability test having a value of 4.26, bursting strength having avalue of 5.4, and filtration efficiency having a value of 14.0 percent.

EXAMPLE 5 Anti-Blocking Agents

Two injection molded disks, 2.5 inch diameter, were placed together witha 94 gram weight on top and held at 50° C. for 24 hours. The disks hadthe following agents compounded therein. The disks were then cooled toroom temperature and pulled apart by hand and ranked for blockingcharacteristics (considerable, slight and none). The following are theresults:

                  TABLE 5                                                         ______________________________________                                        AGENTS                                                                        ______________________________________                                        Poly(lactide) control considerable                                            22% wheat gluten      none                                                    10% wheat gluten      slight                                                  22% pecan shell       none                                                    15% pecan shell       slight                                                  23% wollastonite      slight                                                  28% Ultratalc 609     none                                                    23% Ultratalc 609     none                                                    28% Microtuff F talc  slight                                                  22% Microtuff F talc  slight                                                  14% Microtuff F talc  slight                                                   2% Microtuff F talc  considerable                                            ______________________________________                                    

EXAMPLE 6 Plasticizer Agents

Dried pellets of devolatilized poly(lactide) were processed in a twinscrew extruder to allow compounding various plasticizing agents. Thestrands leaving the extruder were cooled in a water trough and choppedinto Samples of the pellets were heated at 20 C./minutes to 200° C. in aDSC apparatus, held at 200° C. for 2 minutes and rapidly cooled toquench the samples. The quenched samples were then reheated in the DSCapparatus increasing at 20° C./minute to determine the glass transitiontemperature. These samples were compared to a polymer with noplasticizer. The effect of the plasticizer on the glass transitiontemperature is shown in the table below. Glass transition temperaturesare taken at the mid-point of the transition.

                  TABLE 6                                                         ______________________________________                                                                Change in T.sub.g /wt.                                SAMPLE           T.sub.g (C)                                                                          percent additive                                      ______________________________________                                        Control          54.8   --                                                    8% Dioctyl adipate                                                                             35.0   2.5                                                   Control + 40% silica                                                                           54.5   --                                                    Control + 40% silica +                                                                         36.0   3.7                                                   5% dioctyl adipate                                                            Control          54.6   --                                                    6% Citroflex A-4*                                                                              42.6   2.0                                                   12% Citroflex A-4                                                                              31.4   1.9                                                   Control          59.3   --                                                    1.6% Citroflex A-4                                                                             56.3   1.9                                                   2.9% Citroflex A-4                                                                             53.1   2.1                                                   Control          58.4   --                                                    2.1% Citroflex A-4                                                                             56.1   1.1                                                   3.4% Citroflex A-4                                                                             50.5   2.3                                                   Control          61.6   --                                                    18.6% Citroflex A-2                                                                            54.7   0.4                                                   13.1% Citroflex B-6                                                                            52.4   0.7                                                   12.6% Citroflex A-6                                                                            53.8   0.6                                                   ______________________________________                                    

These results show the effectiveness of these plasticizers in reducingthe glass transition temperature of poly(lactide).

The procedure above was tried using corn oil as a plasticizer. Visualobservation showed the corn oil to be not compatible, forming a film onthe surface. Corn oil and mineral oil were both not effective as aprimary plasticizer with poly(lactide). They may still be useful as asecondary plasticizer, in combination with a compatible primaryplasticizer.

EXAMPLE 7 Lactide and Poly(lactide) Equilibrium Concentrations

Experiments were conducted to determine the equilibrium concentration oflactide and poly(lactitemperatureerent temperatures. In theseexperiments a sample of lactide was polymerized in the presence of acatalyst (tin (II) bis(2-ethyl hexanoate)) and held at a fixedtemperature for 18 hours or greater. Beyond this time the residualmonomer concentration is believed essentially constant. The content ofresidual monomer was determined by GPC analysis. GPC analysis wasconducted with an Ultrastyragel® column from Waters Chromatography. Themobile phase was chloroform. A refractive index detector with molecularweight calibration using polystyrene standards was used. The GPCtemperature was 35° C. Data analysis was completed using the softwarepackage Baseline, model 810, version 3.31.

The results of tests conducted on several samples at varioustemperatures are summarized in the graph of FIG. 2 as indicated by X'son such graph. Also plotted on the graph of FIG. 2 are data points citedin A. Duda and S. Penczek, Macromolecules, vol. 23, pp. 1636-1639 (1990)as indicated by circles on the graph. As can be seen from the graph ofFIG. 2, the equilibrium concentration, and thus the driving force behindthe depolymerization of poly(lactide) to form lactide, increasesdramatically with increased temperature. Thus, melt-processing atelevated temperatures results in degradation of the lactide polymer toform lactide on the basis of equilibrium alone. For example, lactideconcentrations below about 2 percent cannot be directly obtained attemperatures of 140° C. or above due to the identified equilibriumrelationship between lactide and poly(lactide).

EXAMPLE 8 Lactide Polymerization in the Presence of a Solid SupportedCatalyst

Tin (II) Oxide

24 grams of L-lactide (melting point about 97° C.) and 6 grams ofD,L-lactide (for the purposes of this invention, D,L-lactide has amelting point of about 126° C.) were combined in a round bottom flaskwith 0.033 grams of Tin (II) oxide, as a fine powder. This correspondsto the catalyst level of 852:1, molar ratio lactide to tin. The flaskwas then purged with dry nitrogen 5 times. This was lowered into an oilbath at 160° C. with magnetic stirring. Polymerization time was 8 hours.

Amberlyst 36

24 grams of L-lactide and 6 grams of D,L-lactide were combined in around bottom flask with 1.06 grams of Amberlyst 36 resin beads. Theflask was purged 5 times with dry nitrogen. The flask was lowered intoan oil bath at 140° C. with magnetic stirring. Polymerization time was 8hours. The resin had a stated proton content of 1 meq/gram dry weightresin. The resin was prepared by rinsing 2 times with 10 volumes drymethanol, then dried for several hours under high vacuum for severalhours at 40° C.

The polymerization results are shown below:

                  TABLE 7                                                         ______________________________________                                        Sample     Mn        Mw      PDI    % Conversion                              ______________________________________                                        Tin (II) Oxide                                                                           77,228    103,161 1.34   54.0                                      Amberlyst   1,112     1,498  1.34   73.5                                      ______________________________________                                    

EXAMPLE 9 Molecular Weight Relationship to Physical Properties ofLactide Polymers

Poly(lactide) samples with various molecular weights and opticalcompositions were prepared by polymerizing blends of L-lactide andmeso-lactide at 180° C. under nitrogen in a 1-gallon sealed reactor.Tin(II) bis(2-ethyl hexanoate) catalyst was added at amonomer-to-catalyst ratio of 10,000:1. After about 1 hour the moltenpolymer was drained from the reactor using nitrogen pressure. The samplewas poured into a pan and placed in a vacuum oven at about 160° C. forabout 4 hours to bring the reaction to near equilibrium levels.

Portions of the samples were then dried under vacuum and processed in aninjection molding apparatus (New Britain 75 from New Britain MachineCo.) to produce standard test bars for physical property testing. Theresults of physical property testing are shown in the following Table 8.The physical property tests were made according to ASTM methods D 638, D256, and D 790. The reported results are the averages of several tests.

Samples of the test bars after injection molding were analyzed by GPCfor molecular weight. Other portions of the test bars were reground andtested in a capillary viscometer to determine the melt-viscosity. Theseresults are also included in Table 8.

Statistical analysis of the data revealed no correlations which werestatistically significant between either optical composition ormolecular weight and the mechanical properties of modulus, tensilestrength, percentage elongation at break, notched Izod impact strength,flexural modulus, or flexural strength. The independence of theseproperties on molecular weight indicates that all of these samples wereabove a "threshold" molecular weight required to achieve the intrinsicproperties of the polymer in a preferred composition.

The viscosity data show significant correlations with molecular weight.This dependence documents the practical limitation and necessity ofcontrolling polymer molecular weight below an upper limit at which it isimpractical to melt-process the polymer. At high molecular weight, highviscosity prevents processing by standard melt-processing equipment.Increases in temperature to reduce viscosity dramatically increasepolymer degradation and lactide formation which is also unacceptable.

                  TABLE 8                                                         ______________________________________                                                    Molecular                                                         Meso        Weight After                                                                            Viscosity at 173° C. (Pa · S)           Sample                                                                              Lactide In                                                                              Injection Final  Shear Rate                                                                           Shear Rate                            I.D.  Blend, Wt %                                                                             Weight    IV (dl/g)                                                                            100 S.sup.-1                                                                         1000 S.sup.-1                         ______________________________________                                        6     40        41000     0.86    5.5   2.9                                   5     10        54000     0.88   10.4   7.2                                   4     20        59000     0.91   10.4   7.2                                   8     10        64000     1.02   15.7   10.0                                  9     40        68000     0.97   12.6   8.1                                   7     20        71000     1.16   36.0   12.9                                  10    20        83000     1.19   35.8   15.8                                  ______________________________________                                        Mechanical Properties of Injection Molded Samples                             Sam- Mod-   Tensile  %      IZOD   Flexural                                                                             Flexural                            ple  ulus   Strength Elongation                                                                           Impact Modulus                                                                              Strength                            I.D. MPSI   (Yld) PSI                                                                              at Break                                                                             ft · lb./in                                                                 MPSI   PSI                                 ______________________________________                                        6    0.55   6600     3.3    0.39   0.53   11300                               5    0.56   7800     3.5    0.46   0.54   12500                               4    0.56   7600     3.9    0.32   0.53   12500                               8    0.55   7700     3.4    0.47   0.53   12400                               9    0.59   6700     3.1    0.42   0.52   10600                               7    0.56   7400     3.3    0.45   0.51   12400                               10   0.55   6700     3.0    0.47   0.52    9900                               ______________________________________                                    

EXAMPLE 10 Effect of Residual Catalyst on Polymer Degradation

Polymer samples were prepared at four levels of catalyst, correspondingto monomer to catalyst molar ratios of 5,000:1, 10,000:1, 20,000:1, and40,000:1. The catalyst utilized was tin (II) bis(2-ethyl hexanoate).These samples were then subjected to heating in a TGA apparatus (TAInstruments, Inc., model 951 thermogravometric analyzer with a DuPont9900 computer support system) with a nitrogen purge. Isothermalconditions of 200° C. for 20 minutes were used. The samples were thenanalyzed by GPC with a viscosity-based detector and a universalcalibration curve to determine the extent of breakdown in molecularweight. The GPC apparatus for this test was a Viscotek Model 200 GPC anda Phenomenex column. The TGA analysis typically resulted in about a 5percent loss in weight and molecular weight drops of 0 to 70 percent.

The number average molecular weights were converted to a milliequivalentper kilogram basis (1,000,000/Mn) in order to calculate a rate of chainscission events. The results below represent averages of 2-4 replicateson each of the four samples.

                  TABLE 9                                                         ______________________________________                                        Catalyst level  Scission Rate                                                 (monomer/catalyst)                                                                            (meq/kg*min)                                                  ______________________________________                                         5,000          1.33                                                          10,000          0.62                                                          20,000          0.44                                                          40,000          0.12                                                          ______________________________________                                    

The rate of chain scission was directly proportional to the residualcatalyst level, demonstrating the detrimental effect of catalystactivity on melt-stability under conditions similar to melt-processing.This instability, however, is distinguished from the instability due tothe equilibrium relationship between lactide and poly(lactide) detailedin Example 7, in that loss of molecular weight due to catalyticdepolymerization by chain scission is evident.

EXAMPLE 11 Catalyst Deactivation Experiment

Two runs were made in a laboratory Parr reactor. Lactide feed was 80percent L-lactide and 20 percent D,L-lactide. Molecular weight wascontrolled by adding a small quantity of lactic acid, the targetmolecular weight was 80,000 Mn.

Lactide was charged to the reactor as a dry mix, the reactor was purged5 times with nitrogen, and heated up to 180° C. At this point catalyst(5000:1 initial monomer to catalyst molar ratio, Fascat® 2003) wascharged through a port in the top of the reactor. The reaction wasallowed to proceed for 70 minutes at 180° C., with mechanical agitation.Conversion at this point was 93-94 percent, close to the equilibriumvalue at 180° C. of 96 percent poly(lactide) from FIG. 2. This point isconsidered tzero, designating the completion of the polymerizationreaction and the beginning of the mixing time.

In the control experiment, a sample was taken and the mixture was heldat temperature with continued agitation. Samples were taken periodicallythrough a port in the reactor bottom. After 4 hours the reactor wasdrained.

In the example experiment, a sample was taken and 0.25 weight percent ofa metal deactivator (Irganox® MD 1024®) was added through the catalystaddition port. The mixture was held at temperature with continuedagitation and samples were withdrawn periodically. The reactor wasdrained after 4 hours.

GPC analysis (utilizing the method of Example 7) for these samples wasdivided into three parts: polymer with molecular weight over 4,000 (forwhich the Mn and Mw numbers are reported), the percent oligomers(comprising the region with molecular weight greater than lactide butless than 4,000, as distinguished from oligomers as defined by Loomis toinclude only oligomers up to a molecular weight of 450), and percentlactide (residual monomer). The structure of the oligomers was notcertain, but it is believed they were primarily cyclic structures. It isalso believed that the metal deactivator, if unreacted, will elute withthe oligomer fraction. Quantification of the oligomer fraction isdifficult, because the GPC trace is near the baseline in this region.

The analysis of the polymer samples as withdrawn from the reactor atvarious time intervals for the control and experimental compositions areshown below in Table 10.

                  TABLE 10                                                        ______________________________________                                        Control                                                                             Mn      Mw       % Polymer                                                                             % Oligomer                                                                            % Monomer                              ______________________________________                                        t-zero                                                                              67,100  119,500  94      0       6.0                                    0.5 hr                                                                              62,500  119,000  95      0.7     3.9                                    1.0 hr                                                                              61,500  116,100  96      0       3.6                                    1.5 hr                                                                              56,000  111,600  95      1.5     3.3                                    2.0 hr                                                                              57,600  110,900  96      0.9     3.1                                    4.0 hr                                                                              51,400  105,400  94      3.3     3.1                                    ______________________________________                                        Test  Mn      Mw       % Polymer                                                                             % Oligomer                                                                            % Monomer                              ______________________________________                                        t-zero                                                                              63,200  110,700  93      3.5     3.8                                    0.5 hr                                                                              52,100  108,600  92      4.6     2.9                                    1.0 hr                                                                              52,700  109,200  92      4.9     2.8                                    1.5 hr                                                                              53,400  107,200  93      4.0     3.1                                    2.0 hr                                                                              59,700  111,100  94      0.6     5.8                                    4.0 hr                                                                              51,200  107,300  91      6.1     3.3                                    ______________________________________                                    

The samples were then ground and placed in a 120° C. oven under vacuum(pressure 0.1 inch Hg) for 14 hours. Sample analyses after thistreatment are shown below in Table 11.

                  TABLE 11                                                        ______________________________________                                        Control                                                                             Mn      Mw       % Polymer                                                                             % Oligomer                                                                            % Monomer                              ______________________________________                                        t-zero                                                                              45,500  88,500   98      2.2     0.0                                    0.5 hr                                                                              45,000  88,700   98      2.0     0.0                                    1.0 hr                                                                              43,900  87,200   98      2.0     0.0                                    1.5 hr                                                                              42,600  84,000   98      2.2     0.0                                    2.0 hr                                                                              42,000  85,200   97      3.2     0.0                                    4.0 hr                                                                              41,900  82,800   98      2.0     0.0                                    ______________________________________                                        Test  Mn      Mw       % Polymer                                                                             % Oligomer                                                                            % Monomer                              ______________________________________                                        t-zero                                                                              39,300  76,700   96      4.0     0.0                                    0.5 hr                                                                              43,900  85,100   98      2.4     0.0                                    1.0 hr                                                                              55,300  98,600   96      3.8     0.0                                    1.5 hr                                                                              48,400  96,200   95      4.5     0.0                                    2.0 hr                                                                              48,900  101,900  95      5.0     0.0                                    4.0   50,600  101,900  94      5.6     0.0                                    ______________________________________                                    

In all cases the polymer was completely devolatilized (0.0 percentresidual lactide monomer). The data also clearly show that the metaldeactivator reduced the degradation of polymer during thedevolatilization step (as indicated by the greater loss in Mn for thecontrol samples from Table 9 to Table 10 versus the Test samples). Onehour of mixing appears to be long enough to develop most of the benefit.

The samples were stored at room temperature under nitrogen for about 1week and reanalyzed, as shown below in Table 12.

                  TABLE 12                                                        ______________________________________                                        Control                                                                             Mn      Mw       % Polymer                                                                             % Oligomer                                                                            % Monomer                              ______________________________________                                        t-zero                                                                              33,500  71,000   100     0.1     0.0                                    0.5 hr                                                                              43,400  95,800   99      1.0     0.0                                    1.0 hr                                                                              44,900  96,300   100     0.1     0.0                                    1.5 hr                                                                              45,900  95,000   100     0.0     0.0                                    2.0 hr                                                                              45,900  94,100   100     0.2     0.0                                    4.0 hr                                                                              43,100  90,100   99      1.3     0.0                                    ______________________________________                                        Test  Mn      Mw       % Polymer                                                                             % Oligomer                                                                            % Monomer                              ______________________________________                                        t-zero                                                                              44,600  84,900   100     0.0     0.0                                    0.5 hr                                                                              45,300  90,600   99      1.2     0.0                                    1.0 hr                                                                              47,800  100,000  98      2.4     0.0                                    1.5 hr                                                                              46,600  98,900   96      3.5     0.0                                    4.0   57,700  110,200  96      4.0     0.3                                    ______________________________________                                    

Equilibrium lactide levels are estimated to be less than 0.2 weightpercent at room temperature. Consistent with that, essentially nolactide was observed in any of the samples (detection limit about 0.1weight percent). The oligomer content in the non-stabilized samplesdeclined and some increase in molecular weight was noted, perhaps due toreincorporation of the (cyclic) oligomers into the polymer. The oligomerdepletion reaction was inhibited in the stabilized polymers, with theextent of inhibition dependent on the length of time that the additivewas mixed.

The samples were then reheated to 180° C. in sealed vials and held forone hour as a simulation of melt-processing. Analysis of the samplesafter the heat treatment is given below in Table 13.

                  TABLE 13                                                        ______________________________________                                        Control                                                                             Mn      Mw       % Polymer                                                                             % Oligomer                                                                            % Monomer                              ______________________________________                                        t-zero                                                                              23,900  60,000   88      8.4     4.0                                    0.5 hr                                                                              23,900  59,600   90      7.7     2.7                                    1.0 hr                                                                              23,700  58,800   88      9.3     2.7                                    1.5 hr                                                                              24,700  58,000   86      10.0    3.8                                    2.0 hr                                                                              26,100  56,400   90      6.8     2.7                                    4.0 hr                                                                              24,800  58,700   92      6.6     1.9                                    ______________________________________                                        Test  Mn      Mw       % Polymer                                                                             % Oligomer                                                                            % Monomer                              ______________________________________                                        t-zero                                                                              33,900  64,300   95      2.2     3.1                                    0.5 hr                                                                              17,900  34,600   94      4.8     1.7                                    1.0 hr                                                                              21,200  42,900   94      4.6     1.8                                    1.5 hr                                                                              29,200  56,900   98      0.5     1.8                                    2.0 hr                                                                              missing                                                                 4.0 hr                                                                              35,700  71,400   95      3.7     1.7                                    ______________________________________                                    

The data for molecular weight show that if the metal deactivator is notmixed into the system long enough then it can have a detrimental impacton stability in the melt. The samples for which the mixing was at least1.5 hours show no detrimental effect, and the 4 hour sample appears tobe somewhat more stable than any of the others based on molecular weightalone. More importantly, the metal deactivator samples showsignificantly less lactide reformation than the control samples. Thiseffect is gained even in the samples which were mixed for only 0.5 hour.The metals deactivated samples averaged only 1.8 percent lactide afterone hour at 180° C., compared to an average of 3.0 percent lactide forthe controls. The equilibrium level at 180° C. is about 3.6 percent fromFIG. 2. Thus, the use of metal deactivators can reduce the troublesomereformation of lactide during melt-processing of the finished polymer.

EXAMPLE 12 Effect of Increased Polymerization Temperature on PolymerCharacteristics

L-lactide (Boeringer Ingleheim, S-grade) was used as received,meso-lactide (PURAC) was purified by distillation to remove traces of D-and L-lactide. The melting point of the purified meso-lactide was 54° C.Lactide mixtures were made up to the following ratios: 100 percentL-lactide, 90/10 L-lactide/meso-lactide, 70/30 L-lactide/meso-lactide,50/50 L-lactide/meso-lactide, and 100 percent meso-lactide. Catalystlevel was 2,500:1 molar ratio of initial monomer to tin with the tinbeing tin(II) bis (2-ethyl hexanoate) (Fascat® 9002). Lactic acid wasadded as a molecular weight control agent to target a number averagemolecular weight of 50,000 (the same amount was added to all samples).Polymerization times were estimated to obtain conversions of 50 percentand 90 percent. For 120° C. this was 4 hours and 16 hours, respectively.For 180° C. these times were 10 minutes and 50 minutes, respectively.Below in Table 14 are the GPC results (method of Example 7) of tests onthe polymer samples produced by this procedure.

                  TABLE 14                                                        ______________________________________                                        L/meso   Temp      Mn       Mw     PDI   % Conv                               ______________________________________                                        100% L   120° C.                                                                          31,014   33,774 1.09  53.2                                                    45,864   52,574 1.15  87.1                                 100% L   180° C.                                                                          27,785   32,432 1.17  46.7                                                    56,839   98,125 1.73  93.3                                 90/10    120° C.                                                                          34,541   38,586 1.12  62.3                                                    29,222   34,466 1.18  89.3                                 90/10    180° C.                                                                          31,632   35,713 1.13  48.5                                                    57,925   110,841                                                                              1.91  94.8                                 70/30    120° C.                                                                          41,211   45,222 1.10  60.1                                                    58,284   71,257 1.22  89.1                                 70/30    180° C.                                                                          32,292   37,401 1.16  53.8                                                    51,245   107,698                                                                              2.10  96.5                                 50/50    120° C.                                                                          15,888   17,969 1.13  57.8                                                    25,539   31,834 1.25  90.6                                 50/50    180° C.                                                                          34,375   42,018 1.22  62.5                                                    44,590   98,028 2.20  95.5                                 100% meso                                                                              120° C.                                                                          33,571   40,635 1.21  73.4                                                    45,237   68,142 1.51  94.3                                 100% meso                                                                              180° C.                                                                          30,976   42,987 1.39  67.6                                                    40,038   83,815 2.09  96.6                                 ______________________________________                                    

The results show that the ultimate number average molecular weight wasnot significantly affected by the temperature of polymerization, with anaverage of 41,000 at 120° C. and 50,000 at 180° C. This implies thateach lactic acid molecule initiates about one polymer chain, regardlessof temperature. The ultimate weight average molecular weight is,however, significantly affected by temperature. At 120° C. the weightaverage molecular weight averaged 52,000 and at 180° C. the was 100,000.This is believed to be due to a relative increase in the rate oftransesterification at 1.80° C. The polydispersity index (PDI) at highconversion also reflects this, averaging 1.3 at 120° C. and 2.0 at 180°C. It is believed these differences would have a significant effect onthe melt-processing characteristics of the polymer, with the higherweight average molecular weight of the polymer produced at 180° C.expected to translate into better melt strength and processability.

These experiments show that polymerization at a higher temperatureresults in a polymer that is characteristically different. Further, theglass transition temperature for the samples polymerized at highertemperature is higher.

EXAMPLE 13 Experiments with Stabilizing Agents and Metal Deactivators

Test 1

Conditions: vial polymerization, (Lactide is melted under anitrogen-purged atmosphere in a round bottom flask with stirring.Catalyst and additives are added and aliquots of the mixtures arepipetted into silanized glass vials. Typically 5-10 grams of reactionmixture are used in a 16 ml. vial. The vials are tightly capped andplaced into a preheated oil bath.) 10,000:1 molar ratio oflactide-to-tin, tin(II) bis(2-ethyl hexanoate) catalyst, 0.2 weightpercent Ultranox® 626 in tetrahydrofuran (THF). 180° C. Time was 90minutes.

The control with tin only polymerized to 84 percent conversion andreached a MWn of 31,700. The example with tin and Ultranox® polymerizedto 83 percent conversion and reached a number average molecular weight(MWn) of 39,800; an increase of 26 percent over the control.

The control sample turned light yellow, the sample with stabilizerremained colorless.

Test 2

Conditions: vial polymerization, 5000:1 molar ratio of lactide to tin,tin(II) bis(2-ethyl hexanoate) catalyst, 0.25 wt percent Ultranox® 626(in THF). 180° C. Time was 60 minutes. Lactide was used from the abovedescribed Gruber et al. process.

The control with tin alone polymerized to 67 percent conversion andreached a MWn of 62,900. The example with tin and Ultranox® polymerizedto 66 percent conversion and reached a MWn of 75800; an increase of 21percent over the control.

A second example with tin(II) bis(2-ethyl hexanoate), Ultranox®, and0.50 percent of Irganox® 1076, which is a phenolic antioxidant,polymerized to 66 percent conversion and reached a number averagemolecular weight (MWn) of 74500; an increase of 18 percent over thecontrol.

All samples were a dark yellow color, although the samples withstabilizer had a slightly lower absorbance at 300 nm.

Test 3

Conditions: vial polymerization, 10,000:1 molar ratio of lactide to tin,tin(II) bis(2-ethyl hexanoate) catalyst, 180° C., 80 percent L-lactideand 20 percent D,L-lactide purchased from Henley and Aldrich,respectively. Lactic acid was added to control molecular weight to about75,000 at full conversion One sample included 0.25 percent Ultranox® 626phosphite stabilizer, one included 0.25 percent Irganox® 1076antioxidant, and one control sample.

Samples were taken at various times and analyzed by GPC for conversionand molecular weight (the method of Example 7). The results aresummarized in Table 15 below.

                  TABLE 15                                                        ______________________________________                                        Time Control      Irganox ®                                                                              Ultranox ®                                 (hrs)                                                                              Mn       % conv  Mn     % conv                                                                              Mn      % conv                             ______________________________________                                        1    31,000   46      35,900 41    66,500  61                                 2    45,400   74      56,800 74    102,700 83                                 4    69,600   93      74,100 93    97,200  91                                 11   52,900   95      60,700 95    71,500  94                                 ______________________________________                                    

The sample with phosphite stabilizer polymerized faster, shown by thehigher conversion at 1 and 2 hours, and went to a higher molecularweight than the control or the sample with Irganox®. The phosphitestabilized sample had a molecular weight more than 30 percent higherthan the control for all time periods.

Test 4

The experiment above was repeated to compare the control to thephosphite-stabilized polymer, as summarized in Table 16 below.

                  TABLE 16                                                        ______________________________________                                        Time     Control         Ultranox ®                                       (hrs)    Mn      % conv      Mn     % conv                                    ______________________________________                                        1        36,600  37          71,500 59                                        2        51,700  70          95,200 85                                        4        64,400  91          103,700                                                                              94                                        8        58,100  96          95,700 94                                        ______________________________________                                    

The sample with phosphite stabilizer again polymerized faster and wentto a higher molecular weight than the non-stabilized sample. Thephosphite stabilized sample had a molecular weight more than 60% higherthan the control for all time periods.

Test 5

Conditions: vial polymerization, 5,000:1 molar ratio of lactide to tin,tin(II) bis(2-ethyl hexanoate) catalyst, 180° C., 80 percent L-lactideand 20 percent D,L-lactide purchased from Henley and Aldrich. Lacticacid was added to control number average molecular weight to anestimated 80,000 at full conversion. One sample was run with 0.25percent Ultranox® 626 phosphite stabilizer, one with 0.25 percentIrganox® 1076 antioxidant, and one control sample.

Samples taken at various times and analyzed by GPC (the method ofExample 1) for conversion and molecular weight. The results aretabulated in Table 17 below.

                  TABLE 17                                                        ______________________________________                                        Time  Control     Irganox ®                                                                              Ultranox ®                                 (hrs) Mn      % conv  Mn     % conv                                                                              Mn     % conv                              ______________________________________                                         1    83,600  76      121,900                                                                              83    162,300                                                                              87                                   4    74,400  93      104,300                                                                              95    123,900                                                                              96                                  24    40,200  96      52,000 96    96,900 97                                  48    34,200  97      30,400 96    56,500 96                                  72    25,000  96      22,400 96    69,500 96                                  ______________________________________                                    

The phosphite-stabilized sample had a molecular weight more than 60percent higher than the control for all time periods. After 72 hours ithad a molecular weight 2.8 times higher than the control. The samplewith antioxidant showed an initial increase in molecular weight,relative to the control, but the effect disappeared after 48 hours.

The phosphite stabilized sample was significantly lighter in color thanthe control or the antioxidant treated sample.

Test 6

Conditions: vial polymerization, 5000:1 molar ratio of lactide to tin,tin(II) bis(2-ethyl hexanoate) catalyst, 0.25 wt percent Ultranox® 626(in THF). 180° C. Time was two hours. Gruber et al. process lactidewashed with isopropyl alcohol was used.

The control with tin alone polymerized to 95 percent conversion andreached a number average molecular weight of 118,000. The example withtin and Ultranox® polymerized to 93 percent conversion and reached anumber average molecular weight of 151,000, an increase of 28 percentover the control.

Test 7

Conditions: vial polymerization at 180° C. 5000:1 molar ratio of lactideto tin, tin(II) bis(2-ethyl hexanoate) catalyst. Lactide was 80 percentL-lactide and 20 percent D,L-lactide, purchased from Henley and fromAldrich. Lactic acid was added to target the molecular weight to an Mnof 80,000. All stabilizers were added at 0.25 weight percent. Molecularweight (number average) was determined for samples pulled at 3 hours,while rate constants were based on samples pulled at 1 hour. The resultsof these screening tests on many stabilizing agents following the aboveprocedure are detailed below in Table 18. Product designations in Table18 are tradenames or registered trademarks.

                  TABLE 18                                                        ______________________________________                                                                             Relative                                 Sample           MWn      % Conversion                                                                             Rate                                     ______________________________________                                        Control 1            65,000   95.9      90                                    Control 2            85,000   95.9     100                                    Control 3            76,000   96.6     100                                    Control 4            69,000   96.2     100                                    Control 5            74,000   96.8     110                                    Control 6            70,000   97.2     110                                    PHOSPHITES                                                                    Ultranox 626                                                                             (GE)      103,000  96.8     100                                    Weston TDP (GE)      64,000   70.0      60                                    Weston PDDP                                                                              (GE)      67,000   76.7      60                                    Weston PNPG                                                                              (GE)      92,000   94.1     100                                    Irgafos 168                                                                              (Ciba-Geigy)                                                                            95,000   95.3     120                                    Weston 618 (GE)      99,000   95.1     100                                    Sandostab P-EPQ                                                                          (Sandoz)  108,000  94.7     110                                    Weston TNPP                                                                              (GE)      88,000   97.9     130                                    PHENOLIC ANTIOXIDANTS                                                         Irganox 1010                                                                             (Ciba-Geigy)                                                                            95,000   97.5     110                                    Cyanox 1790                                                                              (Cyanamid)                                                                              98,000   96.9     120                                    BHT                  87,000   96.5     130                                    Irganox 1076                                                                             (Ciba-Geigy)                                                                            121,000  97.8     130                                    Topanol CA (ICI)     84,000   96.6     160                                    AMINES                                                                        Tinuvin 123                                                                              (Ciba-Geigy)                                                                            65,000   94.8      70                                    Tinuvin 622                                                                              (Ciba-Geigy)                                                                            82,000   95.7      80                                    Naugard 445                                                                              (Uniroyal)                                                                              93,000   98.2     120                                    THIOETHER                                                                     Mark 2140  (Witco)   77,000   97.0     120                                    METAL DEACTIVATORS                                                            Irganox MD1024                                                                           (Ciba-Geigy)                                                                            34,000   65.7      10                                    Naugard XL-1                                                                             (Uniroyal)                                                                              91,000   95.8     110                                    ______________________________________                                    

Note, that with a few exceptions, the phosphites and the phenolicantioxidants provide increased molecular weight with no reduction inpolymerization rate. Of the amines, only Naugard® 445 providedstabilization without a rate decrease. The metal deactivators areexpected to deactivate the catalyst, as was observed for Irganox®MD1024. The Naugard® XL-1 did not accomplish deactivation.

EXAMPLE 14 Polymer Melt Stability as a Function of Moisture Content

Lactide, produced and purified in a continuous (Gruber et al.) process,was fed at a rate of 3 kg/hr to a continuous polymerization pilot plant.Catalyst was added with a metering pump at the rate of 1 part catalystto 5000 parts lactide on a molar basis. The reaction system wasblanketed with nitrogen. The reactor vessels consist of two continuousstirred tank reactors (CSTR) in series. The first had a 1-galloncapacity and the second had a 5-gallon capacity. The reactors were run60-80 percent liquid filled and at 170°-180° C. Polymer melt pumps movedthe liquid from CSTR 1 to CSTR 2, and from CSTR 2 through a die into acooling water trough. The polymer strand thus produced was pulled fromthe trough by a pelletizer and stored as pellets.

The pelletized poly(lactide) was put into a drying hopper and dried at40° C. under flowing dry air. Samples were pulled after one hour andfour hours. These samples were then run through a single screwBrabender® extruder, with a retention time of approximately 3 minutes.Samples were analyzed for moisture by an automatic Karl Fischerapparatus and for molecular weight by GPC (the method of Example 7). Theresults of these tests are documented in Table 19 below.

                  TABLE 19                                                        ______________________________________                                                      Extruder    Weight Average                                      Sample        Temperature (C.)                                                                          Molecular Weight                                    ______________________________________                                        Initial                   63,000                                              Dried 1 hour  137         44,000                                              (1200 ppm H.sub.2 O)                                                                        145         48,000                                                            162         35,000                                                            179         30,000                                              Dried 4 hours 140         63,000                                              (150 ppm H.sub.2 O)                                                                         140         69,000                                                            160         65,000                                                            178         68,000                                              ______________________________________                                    

These results show the detrimental effect of water in the lactidepolymer resin during melt polymerization and the need to properly drythe poly(lactide) before melt-processing.

EXAMPLE 15 Degradation of Crystalline and Amorphous Poly(lactide)

Two literature references disclose poly(D,L-lactide) to degrade fasterthan poly(L-lactide), attributing the result to crystallinity ofpoly(L-lactide). These are: Kulkarni et al., J. Biomed. Mater. Res.,vol. 5, pp. 169-181, (1971); Makino et al., Chem. Pharm. Bull., vol. 33,pp. 1195-1201, (1985). An experiment was conducted to measure the effectof crystallinity on polymer degradation and is detailed below.

An amorphous poly(lactide) sample (clear, and less than 1percentcrystallinity based on DSC) and a crystalline poly(lactide)sample (opaque, and approximately 50 percent crystallinity based on DSC)were subjected to biodegradation in a compost test (50° C., withaeration). The DSC apparatus was a TA Instruments, Inc., model 910differential scanning calorimeter with DuPont 9900 computer supportsystem typically programmed to heating at a rate of 10° C. per minute to200° C. The samples had different optical composition, with thecrystalline sample being, Lore than 90 percent poly(L-lactide) and theamorphous sample being less than 80 percent poly(L-lactide) with thebalance being either poly(D,L-lactide) or poly(meso-lactide). Samples ofeach polymer were subjected to a compost test (ASTM D 5338) whichincluded mixing a stabilized compost and providing a source ofhumidified air while maintaining a temperature of about 50° C. Theamorphous sample was completely degraded after 30 days of composting.The crystalline sample was only 23 percent degraded based on carbondioxide after the same period of time.

Additional samples of these two polymers were subjected to chemicalhydrolysis at 50° C. (hydrolysis is believed to be the rate-limitingstep in the biodegradation process). The chemical hydrolysis procedureincluded placing 0.1 gram poly(lactide) in 100 ml of 0.2M phosphatebuffer (pH 7.4). The samples were held for 1 week, then filtered, washedwith deionized water, and dried at 25° C. under vacuum. The initialweight average molecular weight for each sample was about 70,000. After1 week the amorphous had a weight average molecular weight of 10,000 andcrystalline sample had a weight average molecular weight of 45,000,determined by GPC (the method of Example 7). Neither sample hadsignificant weight loss at this time.

Both of these tests demonstrate that degradation of crystallinepoly(lactide) is slower than degradation of amorphous poly(lactide).

EXAMPLE 16 Effect of Monomer Concentration on Film Modulus

Poly(lactide) was precipitated in methanol from a chloroform solution inorder to remove the residual lactide monomer. GPC analysis (the methodof Example 1) showed the precipitated polymer to contain 0.0percentlactide.

The polymer was dissolved in chloroform to make a 10 wt percentsolution, and lactide was added back to make 5 separate solutions which,after removing the chloroform, are calculated to produce filmscontaining 0.0, 0.2, 0.4, 1.0 and 4.0 weight percent lactide inpoly(lactide). These solutions were solvent cast onto glass, driedovernight at room temperature in a fume hood, and removed to a vacuumoven. The films were hung in the vacuum oven and dried at 30° C. for 72hours. GPC analysis of the vacuum-dried films showed measured lactidelevels of 0.0, 0.0, 0.4, 0.7 and 3.7 wt percent.

The films were then tested for film modulus using ASTM procedure D882.

The results are shown below in Table 20.

                  TABLE 20                                                        ______________________________________                                                                           Elastic                                    %      Tensile  Std.   %      Std. Modulus                                                                              Std.                                Lactide                                                                              (psi avg.)                                                                             Dev.   Elongation                                                                           Dev. (psi avg.)                                                                           Dev.                                ______________________________________                                        0      5490     636    2.85   0.14 730,000                                                                              103,000                             0      6070     123    2.85   0.22 818,000                                                                              35,000                              0.4    5670     227    2.75   0.27 779,000                                                                              44,000                              0.7    5690     343    4.04   1.12 749,000                                                                              58,000                              3.7    5570     458    3.33   1.43 738,000                                                                              66,000                              ______________________________________                                    

EXAMPLE 17 Rate of Water Uptake Versus Optical Composition

Samples of poly(lactide), made from 80 percent L-lactide and 20 percentof either D,L-lactide or meso-lactide, were ground to pass a 20 meshscreen. The samples were dried and devolatilized under vacuum thenremoved to a constant humidity chamber maintained at 24° C. and 50percent relative humidity. The rate of moisture pick-up was determinedgravimetrically, with the final results verified by Karl-Fischer wateranalysis. The rate of moisture pickup is shown below in Table 21.

                  TABLE 21                                                        ______________________________________                                                       Parts per Million                                              Time           Weight Gain                                                    (Minutes)      L/D,L Polymer                                                                            L/Meso Polymer                                      ______________________________________                                        10              600       1000                                                30             1100       1500                                                60             1500       1800                                                120            1600       2100                                                870            2100       2600                                                Final (Karl-Fischer)                                                                         3000       2600                                                ______________________________________                                    

EXAMPLE 18 Standard Test of Melt Stability

A standard test for determining melt stability is as follows:

A small sample (200 grams or less) of polymer is ground or pelletizedand devolatilized by holding under vacuum (about 10 mm Hg) at atemperature of 130° C. or less for 18 hours. At this point the residuallactide content should be 1 wt percent or less. Portions (1-5 grams) ofthe devolatilized sample are then placed in a 16 ml sample vivial,tightly capped, and placed in a 180° C. oil bath. Samples are removed attimes of 15 minutes and 1 hour and analyzed for lactide content by GPCor other appropriate techniques. Lactide which may collect on the coolerportions of the vial is included in the product work-up and test.

Melt-stabilized poly(lactide) will show less than 2 percent lactide inthe 15 minute sample, and more preferably less than 2 percent lactide inthe 1 hour sample. The most highly stabilized poly(lactide)s willmaintain lactide contents of less than 1 percent in both the 15 minuteand 1 hour samples, preferably less than 0.5 percent. An unstabilizedpoly(lactide) may reach the equilibrium lactide content at 180° C. of3.6 wt percent, or may go even higher as lactide is driven from thepolymer melt and collects on the cooler top walls of the vial.

EXAMPLE 19 Water Scavenger Experiments

Dried poly(lactide) pellets were processed in a twin screw extruder todevolatilize and to prepare a portion with 0.5 percent by weight of awater scavenger (Stabaxol® P). The strands leaving the extruder arecooled in a water and chopped into pellets. Samples of the control andthe test sample were then analyzed by the Karl Fischer technique formoisture content, with no drying. The control sample contained 1700 ppmwater, the test sample had 450 ppm water. The control sample was thendried under nitrogen at 40° C., reducing the water content to 306 ppm. Avacuum-dried control sample had 700 ppm water.

The as-produced test sample and the dried control samples were thenprocessed in a 1/2" single screw extruder (Brabender®) at 160° C., witha retention time of 3 minutes. The number average molecular weight forthe dried control sample dropped from an initial value of 44,000 to afinal value of 33,000 for the 306 ppm water sample and to 28,000 for the700 ppm water sample. The test sample number average molecular weightdropped from an initial value of 40,000 to a final value of 33,000.

This sample shows how the water scavenger protected the polymer frommoisture pick-up, imparting the same stability as a thorough drying ofthe control sample. Combining a water scavenger with appropriate dryingis expected to give even greater stability. cl EXAMPLE 20

Optimization of Catalyst Concentration

A mixture of 80 percent L-lactide and 20 percent D,L-lactide waspolymerized using three different levels of tin(II) bis(2-ethylhexanoate) catalyst. Batches were prepared at initial monomer/catalystmolar ratios of 1000:1, 3000:1, and 20,000:1. polymerization times wereadjusted to reach high conversion without being excessively long andthereby causing degradation in the melt. The reaction times were 1,2 and20 hours, respectively. The polymerization temperature was 180° C. Thepolymers were ground to a coarse powder and devolatilized at 125° C. and10 mm Hg overnight. The samples were then reground and 1-gram portionsof each were placed into silanized vials, 16 ml capacity. The vials weresealed and placed into an oil bath at 180° C. Vials were then removed atvarious times and the samples were analyzed by GPC after dissolution inchloroform. The molecular weights and lactide contents are shown belowin Table 22.

                  TABLE 22                                                        ______________________________________                                               Time   Number Average                                                                             Weight Average                                                                          Lactide                                  Sample (min)  Molecular Weight                                                                           Molecular Weight                                                                        Weight %                                 ______________________________________                                        1000:1  0     39,000       81,300    0.8                                              5     28,100       57,300    2.4                                             15     25,800       49,700    2.8                                             30     23,100       43,800    3.7                                             60     22,800       43,200    3.6                                      3000:1  0     53,100       113,600   0.6                                              5     39,000       76,400    0.4                                             15     30,300       65,400    1.9                                             30     29,000       60,400    2.7                                             60     28,200       55,200    2.8                                      20000:1                                                                               0     89,200       184,000   0.0                                              5     81,200       165,100   0.0                                             15     54,300       134,600   0.1                                             30     51,100       119,600   0.0                                             60     49,500       111,000   0.0                                      ______________________________________                                    

These results show the benefit of optimizing the catalyst level used inthe polymerization process. Note that both lactide reformation andmolecular weight retention benefits are realized from the reducedcatalyst levels (higher monomer/catalyst ratio).

It is believed catalyst levels should be limited to 1000:1 for the highend of catalyst usage, with 3000:1 being more preferable and showingsomewhat improved stability. Lower levels still, such as 20000:1, showgreatly improved stability. Beyond this level it is believed thepolymerization rates become too slow to be practical.

EXAMPLE 21 Removal of Tin Catalyst from Poly(lactide) by Precipitation

45 grams of L-lactide and 13 grams of D,L-lactide were charged with 78milligrams of crystalline lactic acid to a 200 ml round bottom flask.This was heated to 180° C. with magnetic stirring in an oil bath andblanketed with dry nitrogen. Catalyst in the form of tin(II) bis(2-ethylhexanoate) was added as 0.20 ml of a 0.47 g/ml solution in THF after themolten lactide was at temperature. The mixture was allowed to stir forone minute and then pipetted into 3 silanized glass vials, which werethen sealed and placed into a 180° C. oil bath for 75 minutes. The vialswere allowed to cool and the polymer recovered by breaking the glass.The polymer was ground to a coarse powder and dissolved in chloroform tomake a 10 percent solution. The polymer contained 3.8 percent residualmonomer and had a number average molecular weight of 70,000 asdetermined by GPC measurement (the method of Example 9). 500 ml ofmethanol were placed in a 1-liter glass blender flask. The blender wasturned on to medium speed and 50 ml of the polymer in chloroformsolution was poured in over a period of three minutes. After oneadditional minute of blending the mixture was filtered, then rinsed with100 ml of methanol, and dried overnight under vacuum. The polymerconsisted of a fibrous mat. It contained 0.3 percent residual monomerand had a number average molecular weight of 66,900.

The measured tin level in the precipitated polymer was 337 ppm byweight, compared to a calculated value of 466 ppm for the as-producedpolymer. This result indicates the feasibility of reducing residualcatalyst levels in lactide polymers by solvent precipitation with thebenefit of improved stability as detailed in Example 20.

EXAMPLE 22

Samples of devolatilized poly(lactide) were tested in a Rosand Model 14°C. capillary rheometer. The die was 1 mm diameter and 16 mm long, withan entry angle of 180°. The table below gives the pressure drop acrossthe die as a function of nominal shear rate (not Rabinowitsch corrected)for various molecular weights and temperatures.

                  TABLE 23                                                        ______________________________________                                                                      Nominal                                                                              Pressure                                                               shear rate                                                                           Drop                                     Mn        MW      Temp. (°C.)                                                                        (s.sup.-1)                                                                           (MPa)                                    ______________________________________                                        Results at 150 ° C.                                                    34,000     70,000 150          192    2.0                                                                    384    5.5                                                                    960   10.0                                                                   1920   13.8                                                                   4800   19.7                                                                   9600   23.7                                     52,000    108,000 150          192    9.9                                                                    384   15.6                                                                    960   19.9                                                                   1920   23.9                                                                   4800   29.4                                                                   9600   --                                       60,000    137,000 150          192    7.4                                                                    384   11.1                                                                    960   16.6                                                                   1920   21.0                                                                   4800   --                                                                     9600   --                                       183,000   475,000 150          192   19.1                                                                    384   27.0                                                                    960   31.4                                                                   1920   --                                                                     4800   --                                                                     9600   --                                       Results at 175° C.                                                     34,000     70,000 175          192    0.4                                                                    384    0.5                                                                    960    3.4                                                                   1920    5.5                                                                   4800    9.2                                                                   9600   12.5                                     52,000    108,000 175          192    2.2                                                                    384    4.6                                                                    960    7.6                                                                   1920   11.5                                                                   4800   17.2                                                                   9600   22.1                                     183,000   475,000 175          192   11.5                                                                    384   16.6                                                                    960   20.2                                                                   1920   24.4                                                                   4800   29.9                                                                   9600   --                                       Results at 200° C.                                                     60,000    137,000 200          192    0.5                                                                    384    1.6                                                                    960    3.3                                                                   1920    5.3                                                                   4800   --                                                                     9600   13.2                                     183,000   475,000 200          192    7.0                                                                    384   11.0                                                                    960   14.2                                                                   1920   17.9                                                                   4800   21.6                                                                   9600   --                                       ______________________________________                                    

EXAMPLE 23 Effect of Meso-lactide Concentration on Rate ofCrystallization

Polymer samples of various optical composition were prepared bypolymerizing mixtures of L-lactide and meso-lactide with Tin IIbis(2-ethyl hexanoate) catalyst at a temperature of about 180° C. Aportion of each sample was tested in a Mettler Differential ScanningCalorimeter Model 30 (DSC) by heating from 20° C. to 200° C. at 20°C./minute. The sample was then held at 200° C. for 2 minutes tocompletely melt any crystals. The sample was thereafter rapidly quenchedand reheated with the same procedure. The rapid heat-ups in this methodallow a limited time for crystallization to occur, allowing differencesin crystallization rates to be observed. Results are shown in the tablebelow.

                  TABLE 24                                                        ______________________________________                                                  Exotherm Peak Temp.                                                                              Endotherm                                                                             Peak Temp.                               Sample % meso                                                                           (J/gm)   (°C.)                                                                            (J/gm)  (°C.)                             ______________________________________                                        First upheat                                                                  0         29.1     114       33.7    172                                      3         44       126       5.9     159                                      6         0        --        0       --                                       9         0        --        0       --                                       Second upheat                                                                 0         14.1     137       12.2    173                                      3         0        --        0       --                                       6         0        --        0       --                                       9         0        --        0       --                                       ______________________________________                                    

The results show that the rate of crystallization for the polymer isdecreased dramatically with the addition of small amounts ofmeso-lactide to the polymerization mixture.

EXAMPLE 24 The Effect of Meso-lactide Concentration on Crystallization

Samples of devolatilized poly(lactide) of varying optical compositionand with number average molecular weights in the range of 50,000 to130,000 were prepared in a continuous pilot plant. The samples weredissolved in chloroform to a concentration of 5 grams/100 cc and theoptical rotation of the samples was measured to determine theconcentration of meso-lactide which had been present in the monomermixture prior to polymerization. Separate optical rotation and gaschromatography analysis of the monomer mixture confirmed that L-lactideand meso-lactide are the predominate components when meso-lactide ispresent at a concentration of 20 percent or less, and only a smallcorrection is required for D-lactide.

Additional samples were made by polymerizing mixtures with known weightsof L-lactide and meso-lactide.

All samples were subjected to an annealing procedure to developcrystallinity. The annealing procedure consisted of placing the samplesin an oven at 100°-105° C. for 90 minutes, then lowering the temperature10° C. each 1/2 hour until the temperature reached 45° C. The oven wasthen shut off and the samples were allowed to cool to room temperature.The energy of the melting endotherm and the peak melting temperaturewere then measured using a Mettler Differential Scanning Calorimeter(DSC) apparatus with a scan speed of 20° C. minute. The energy ofmelting is a measure of crystallinity in the annealed samples.

FIG. 3 shows the sharp decline in potential crystallinity between 9 and12 percent meso content.

In contrast, a polymer sample made by polymerizing 80 percent L-lactideand 20 percent D,L-lactide showed, after annealing, a melting endotheimof 12.3 J/gm. This composition has the same enantiomeric excess in termsof lactide acid R- and S- unit as does an 80 percent L-lactide/20percent meso-lactide blend. The 20 percent meso-lactide containing blendshowed no crystallinity after annealing, as shown by FIG. 3.

EXAMPLE 25 Effect of Plasticizer on Crystallization Rate

Devolatilized polymer samples from a continuous pilot plant werecompounded with dioctyl adipate (a plasticizing agent) and/or silicawith a twin screw extruder. The samples were then tested for nucleationrates using the DSC method of Example 23. The table below shows thatdioctyl adipate (DOA) can increase the rate of crystallization ofpoly(lactide) or of a filled poly(lactide).

                  TABLE 25                                                        ______________________________________                                                  Exotherm Peak Temp.                                                                              Endotherm                                                                             Peak Temp.                               Sample    (J/gm)   (°C.)                                                                            (J/gm)  (°C.)                             ______________________________________                                        Base polymer                                                                            0        --        1.3     143                                      Base polymer +                                                                          24.6     84        22.1    147                                      8 wt % DOA                                                                    Base polymer +                                                                          2.4      86        3.9     149                                      40 wt % silica +                                                              Base polymer +                                                                          14.9     86        15.4    147                                      40 wt % silica +                                                              5 wt % DOA                                                                    Second upheat                                                                 Base polymer                                                                            0        --        0       --                                       Base polymer +                                                                          25.0     98        24.0    143                                      8 wt % DOA                                                                    Base polymer +                                                                          0        --        0       --                                       40 wt % silica +                                                              Base polymer +                                                                          15.2     97        14.6    143                                      40 wt % silica +                                                              5 wt % DOA                                                                    ______________________________________                                    

EXAMPLE 26 An Evaluation of Nucleating Agents

A devolatilized sample of poly(lactide) polymer was compounded with avariety of potential nucleating agents in a single screw extruder. Thecandidate nucleating agents were added at a nominal level of 5 percentby weight. The single screw extruder is not as effective of a mixer aswould be used commercially, so failure to observe an effect in thesetests does not mean that a candidate agent would not be effective ifblended more thoroughly. A positive result in this test demonstratespotential ability to increase crystallization rates. Additives whichincreased crystallinity in the second upheat (on a quenched sample) wererated ++, additives which showed an effect only on the first upheat wererated +, and additives which showed no effect were rated 0.

                  TABLE 26                                                        ______________________________________                                        Additive                Effect                                                ______________________________________                                        None                    0                                                     talc, MP1250 (Pfizer)   ++                                                    3-nitro benzoic acid    0                                                     saccharin, sodium salt  ++                                                    terephthalic acid, disodium salt                                                                      0                                                     calcium silicate, -200 mesh                                                                           +                                                     sodium benzoate         +                                                     calcium titanate, -325 mesh                                                                           +                                                     boron nitride           +                                                     calcium carbonate, 0.7 micron                                                                         0                                                     copper phthalocyanine   +                                                     saccharin               0                                                     low molecular weight polyethylene                                                                     0                                                     talc, Microtuff-F (Pfizer)                                                                            ++                                                    talc, Ultratalc (Pfizer)                                                                              ++                                                    ethylene acrylic acid sodium ionomer                                                                  0                                                     (Allied Signal)                                                               isotactic polypropylene +                                                     polyethylene terephthalate                                                                            0                                                     low molecular weight poly(L-lactide)                                                                  ++                                                    Millad 3940 (Milliken)  ++                                                    Millad 3905 (Miliken)   +                                                     NC-4 (Mitsui)           +                                                     polybutylene terephthalate                                                                            +                                                     talc in polystyrene (Polycom Huntsman)                                                                +                                                     talc in polyethylene (Advanced Compounding)                                                           ++                                                    ______________________________________                                    

EXAMPLE 27 Orientation and Rate of Crystallization

DSC was used to determine the effectiveness of orientation as a methodof increasing the rate of crystallization. The method used is the sameas in Example 23. An oriented sample will increase crystallization rateprimarily on the first upheat. The second upheat, which is on a samplethat has been melted and quenched and therefore is no longer oriented isnot expected to show crystallization. The results in the table belowshow an increase in crystallization rate for the nonwoven fibers ofExamples 3 and 4. The melting and quenching procedure (heating at 200°C. for 2 minutes, followed by rapid cooling) reduced the crystallizationrate, although the effect of orientation did not disappear. It isbelieved that a longer hold time in the melt would eliminate the effectof orientation.

                  TABLE 27                                                        ______________________________________                                                  Exotherm Peak Temp.                                                                              Endotherm                                                                             Peak Temp.                               Sample    (J/gm)   (°C.)                                                                            (J/gm)  (°C.)                             ______________________________________                                        First upheat                                                                  Feed pellet                                                                             0        --        0.6     147                                      from Example 3                                                                Nonwoven, 19.4     120       19.7    150                                      Example 3                                                                     Feed pellet                                                                             0        --        0.2     169                                      from Example 4                                                                Nonwoven, 22.8     118       21.5    148                                      Example 4                                                                     Second upheat                                                                 Feed pellet                                                                             0        --        0       --                                       from Example 3                                                                Nonwoven, 10.5     127       9.6     148                                      Example 3                                                                     Feed pellet                                                                             0        --        0       --                                       from Example 4                                                                Nonwoven, 7.1      125       7.5     146                                      Example 4                                                                     ______________________________________                                    

It will be understood that even though these numerous characteristicsand advantages of the invention have been set forth in the foregoingdescription, together with details of the structure and function of theinvention, the disclosure is illustrative only, and changes may be madein detail, especially in matters of shape, size and arrangement of theparts or in the sequence or the timing of the steps, within the broadprinciple of the present invention to the full extent indicated by thebroad general meaning of the terms in which the appended claims areexpressed.

What is claimed:
 1. A nonwoven fabric comprising:(a) a plurality offibers, at least a first portion of which comprise fibers including afirst melt stable polymer composition having a number average molecularweight of from about 10,000 to about 300,000;(i) said first polymercomposition having a lactide monomer concentration of less than about 2%by weight; (ii) said first polymer composition being formed from atleast meso-lactide and remaining lactide; said remaining lactide beingselected from the group consisting of L-lactide, D-lactide and mixturesthereof; and (iii) said. first polymer composition being formed from areaction mixture comprising about 0.5% to about 50% by weightmeso-lactide based on the total amount of meso-lactide and remaininglactide; (b) said first polymer composition being in a form such that anisolated and devolatilized sample, prior to incorporation into saidfibers, would form less than 1% by weight lactide upon heating at 180°C. for 1 hour.
 2. A nonwoven fabric according to claim 1 wherein:(a)said first polymer composition is formed from at least: meso-lactide;remaining lactide; and, epoxidized oil.
 3. A nonwoven fabric accordingto claim 2 wherein:(a) said first melt stable polymer composition has anumber average molecular weight of from about 50,000 to about 250,000.4. A nonwoven fabric according to claim 2 wherein:(a) said first polymercomposition is formed from a reaction mixture comprising about 12% toabout 50% by weight meso-lactide, based on the total amount ofmeso-lactide and remaining lactide.
 5. A nonwoven fabric according toclaim 2 wherein:(a) said first melt stable polymer composition has awater concentration, if water is present at all, of less than about 500parts per million.
 6. A nonwoven fabric according to claim 2 wherein:(a)said first polymer composition is formed from meso-lactide; remaininglactide; epoxidized oil; and, antioxidant stabilizing agent.
 7. Anonwoven fabric according to claim 2 wherein:(a) said first portion offibers comprise melt blown fibers.
 8. A nonwoven fabric according toclaim 2 wherein:(a) said first portion of fibers comprise spunbondfibers.
 9. A nonwoven fabric according to claim 2 wherein:(a) said firstpolymer composition includes organic filler.
 10. A nonwoven fabricaccording to claim 2 wherein:(a) said first polymer composition includespigment.
 11. A nonwoven fabric according to claim 2 wherein:(a) saidfirst portion of fibers comprise a semi-crystalline polymer composition.12. A nonwoven fabric according to claim 11, wherein:(a) said firstpolymer composition is formed from a reaction mixture comprising lessthan about 12% by weight meso-lactide, based on the total amount ofmeso-lactide and remaining lactide.
 13. A nonwoven fabric according toclaim 1 wherein:(a) said first polymer composition is formed from:meso-lactide, remaining lactide and at least one other component capableof reacting into the polymer.
 14. A nonwoven fabric according to claim13 wherein:(a) said at least one other component capable of reactinginto the polymer comprises molecules having at least two sites forreaction into the polymer.
 15. A nonwoven fabric according to claim 14wherein:(a) said molecules having at least two sites for reaction intothe polymer include molecules having at least three sites for reactioninto the polymer.
 16. A nonwoven fabric according to claim 15wherein:(a) said component having molecules with at least three sitesfor reaction to the polymer comprises a triol.
 17. A nonwoven fabriccomprising:(a) a plurality of fibers, at least a first portion of whichcomprise fibers formed from a melt stable first polymer compositionhaving a number average molecular weight of from about 10,000 to about300,000;(i) said first polymer composition having a lactide monomerconcentration of less than about 2% by weight; (ii) said first polymercomposition including polymer chains comprising monomer units derivedfrom at least meso-lactide; and, L-lactide; and, (iii) said firstpolymer composition being formed from a reaction mixture comprisingabout 0.5% to about 50% by weight meso-lactide, based on the totalamount of meso-lactide, L-Lactide and, if present, D-lactide; and (b)said first polymer composition being in a form such that an isolated anddevolatilized sample of it, prior to incorporation into said fibers,would form less than 1% by weight lactide, upon heating at 180° C. for 1hour.
 18. A nonwoven fabric according to claim 17 wherein:(a) saidpolymer composition includes polymer chains comprising units derivedfrom at last one additional material to meso-lactide and L-lactide. 19.A nonwoven fabric according to claim 18 wherein:(a) said at least oneadditional material is D-lactide.
 20. A process for preparing a nonwovenfabric; said process including steps of:(a) melt processing a meltstable first polymer composition into fibers; said first polymercomposition having a number average molecular weight of from about10,000 to about 300 000;(i) said first polymer composition, during saidstep of melt processing, having a lactide concentration of less thanabout 2% by weight and a concentration of water, if present at all, ofless than about 2000 parts per million; (ii) said first polymercomposition being formed from at least meso-lactide and remaininglactide; said remaining lactide being selected from the group consistingof L-lactide, D-lactide and mixtures thereof; said polymer compositionbeing formed from a reaction mixture comprising about 0.5% to about 50%by weight meso-lactide, based on the total amount of meso-lactide andremaining lactide; and, (iii) said step of melt processing a melt stablefirst polymer composition further comprising melt processing a polymercomposition formed from epoxidized oil; and, (b) forming the fibers,made from melt processing of the melt stable polymer composition, into anonwoven fabric.
 21. A process according to claim 20 wherein:(a) saidstep of melt processing to form fibers comprises a step of melt blowingfibers.
 22. A process according to claim 20 wherein:(a) said step ofmelt processing to form fibers comprises a step of forming spunbondfibers.